Tuning an AR....

[BigBore44]

Why do you think the carrier velocity is excessive? (Bolt thrust is a very different thing). The buffer does have a rubberized bumper for a reason. Are you getting any other signs of excessive carrier speed other than feeling like the reciprocating mass is whacking the end of the receiver extension (like it’s supposed to)?

If you say that’s what it’s supposed to do, then I’ll take your word for it. The recoil of the rifle isn’t unpleasant. It’s just like I feel two recoils. One when the round goes off. The second when the carrier whacks the end of the receiver extension. My AR-15 doesn’t do this. Nor do my AR-47’s. But it is a different animal I suppose. I haven’t had time to disassemble the rifle to look and see if there’s any damage. Not entirely sure where to look. I guess I’ll look around and see if there is any coating missing.

I’m not so much concerned that there is a problem. I just don’t know how to determine if there is one. As a nurse, WebMD is the devil. No hypochondria here.

 

[silicosys4]

Your bolded statement is non-sequitur. The compressive force to which the buffer is exposed is not uniquely dependent upon the carrier weight, and the rest of the “symptoms” he’s speculating either aren’t caused by excessive carrier speed at all, or aren’t ONLY caused by said.

Example: Dented brass on the case mouth has next to nothing to do with carrier speed - AR’s do not have a fixed ejector, it’s a spring ejector - so increasing carrier speed does not increase the ejection force. Dented case mouths are over strength ejectors pressing the case mouth against the extension during extraction. If that over strength ejector is over powering the extractor too early during the bolt stroke, it can fall limp in the action before it clears the port, causing a double feed or overbolt jam (really raunchy double feed), or it might cause a stovepipe if it does clear the port and leave the case head in the way of the closing bolt. But equally - an overstrength extractor can cause stovepipes as well, as the extractor doesn’t release the round readily to clear the bolt travel.

What you might be confusing here - dents in the case body below be shoulder where the round is bouncing off of the brass deflector (as it is designed to do) are typically an example of excessive carrier speed. The ejector is doing its job, as in flicking the case out of the port, but the carrier is pulling the case rearward so quickly, it cartwheels around and smashes its shoulder into the deflector. The faster the carrier, the faster the case is moving rearward, the faster it’s moving, the harder it hits the deflector. That’s a different symptom than dented mouths, squished by an overpowered ejector spring.

Not getting “rounds to cycle out of the magazine” might describe symptom of a multitude of operating issues; an undergassed or over sprung AR which is short stroking may not cycle rounds out of the magazine because it doesn’t open far enough to pick up the next round - leaving either an empty chamber, or a bolt-over jam. An overgassed or undermassed AR might outrun the mag and also leave the chamber empty or cause a bolt-over, but it also might just be an out of spec mag catch or out of spec mag leaving rounds too low in the receiver for pick up... Could be under powered ammo causing a short stroke, or a gas system leak, or excessive drag on the carrier, or over length buffer spring, or a plugged RE vent... or the OP may have said “it won’t feed from the mag,” generically, describing what is really a double feeding issue, described above, which has its own systemic solution.

So instead of speculating based on any particular source of a mechanical failure, I asked the OP to better define the symptoms, specifically why he thought he had a “bolt thrust” issue, other than feeling like the buffer was bouncing off of the RE (like it’s supposed to).

If his particular combination of symptoms are real, then a systematic troubleshooting of these symptoms will generally yield the singular solution.

But I rather expect this is nothing more than a thought experiment kind of akin to a hypochondriac perusing WebMD and determining they have EVERYTHING disease... AR’s with multiple concurrent/coincidental, independent failures do happen, but more often, it’s just one issue, and the troubleshooting path is short and sweet.

Lots of good info there, and a good breakdown of your explanation, thanks.
Although I'm not entirely convinced the 35% greater weight of the .308 carrier over a 5.56 carrier, and subsequent overcompression of the buffer stop in an overgassed 308 gun does not contribute to function issues, I have no personal or direct evidence to support that, so I'll keep that one to myself until I have more info.

 

[silicosys4]

If you say that’s what it’s supposed to do, then I’ll take your word for it. The recoil of the rifle isn’t unpleasant. It’s just like I feel two recoils. One when the round goes off. The second when the carrier whacks the end of the receiver extension. My AR-15 doesn’t do this. Nor do my AR-47’s. But it is a different animal I suppose. I haven’t had time to disassemble the rifle to look and see if there’s any damage. Not entirely sure where to look. I guess I’ll look around and see if there is any coating missing.

Two obvious points of damage I would look for are the lower receiver for carrier marks,
and the buffer itself for mushrooming.

Below is commonly seen, mild damage, cosmetic only, and easy to fix. If your carrier is impacting hard enough to cause damage, this would be the first place I would expect to see evidence. The solution is to either recontour the lip of the buffer tube so it can be screwed in one more turn, giving you a little less depth in the buffer tube which will limit carrier over travel, or to remove the buffer and spring and drop a quarter or two into the tube, depending on what you need. Make sure you watch the distance between your bolt face at full recoil and your bolt catch. You want somewhere between 1/4" and 3/16".



Below is severe damage caused by a buffer that is too short for the buffer tube. As you can see the displacement of material into the threads of the receiver ring has changed the shape of the ring itself and damaged the buffer tube. This receiver is most likely trashed, IMO. This happened because of either gross negligence at the factory, or a builder selecting the wrong parts. This isn't an out of spec parts or assembly problem as in the previous photo, this is a parts selection problem. The buffer wasn't impacting the buffer tube at all, the carrier was impacting the receiver instead because the buffer was too short.



And the Buffer for signs of mushrooming. Notice the flared end where the buffer bumper meets the buffer body.




Edit: If you have a collapsible stock, you can also take the stock off and inspect the end cap of the buffer tube with a straight edge for signs of bulging or rounding

 

[Matt.Cross]

If you say that’s what it’s supposed to do, then I’ll take your word for it. The recoil of the rifle isn’t unpleasant. It’s just like I feel two recoils. One when the round goes off. The second when the carrier whacks the end of the receiver extension. My AR-15 doesn’t do this. Nor do my AR-47’s. But it is a different animal I suppose. I haven’t had time to disassemble the rifle to look and see if there’s any damage. Not entirely sure where to look. I guess I’ll look around and see if there is any coating missing.

I’m not so much concerned that there is a problem. I just don’t know how to determine if there is one. As a nurse, WebMD is the devil. No hypochondria here.

You could try a "zero sum" change: a muzzle brake. Reduces felt recoil without introducing any mechanical changes. Then if that doesn't work, go to minor mechanical changes, recoil spring, buffer weight, and adjustable gas block. That is the order I would personally make changes in, but that's just me.

 

[Varminterror]

Lots of good info there, and a good breakdown of your explanation, thanks.
Although I'm not entirely convinced the 35% greater weight of the .308 carrier over a 5.56 carrier, and subsequent overcompression of the buffer stop in an overgassed 308 gun does not contribute to function issues, I have no personal or direct evidence to support that, so I'll keep that one to myself until I have more info.

Here’s the fallacy....

You - like many - assume a 308win is far more powerful in terms of energy and momentum, so the force exerted upon the reciprocating mass must be proportionately more powerful. But the reciprocating mass isn’t exposed to the energy or momentum of the cartridge, it’s exposed to the gas pressure at the port position (minus fitting losses in the gas system, naturally). The gas tubes are the same diameter, so the cross-sectional area is the same, and the gas port diameters recommended for both are similar (~.096” for 20” rifle length) - the only difference could be the pressure... but wait... 5.56x45 EPVAT standard is 62kpsi and change, while EPVAT for 7.62x51mm is 60kpsi and change. Looking at the pressure curve for each, and referencing the pressure at any gas port locations, you can see the resulting force on each action really isn’t so different: since Force = Pressure * Area and both pressures and areas are the same:



So you assume the 35% increased mass isn’t sufficient, but don’t look at any objective science to suggest WHY it’s insufficient - if the operating forces are so similar... seems like you just speculate a 2800ft.lb. rifle must operate much more violently than a 1300ft.lb. rifle, without understanding the actual forces involved.

 

[silicosys4]

Here’s the fallacy....

You - like many - assume a 308win is far more powerful in terms of energy and momentum, so the force exerted upon the reciprocating mass must be proportionately more powerful.

That's not at all what I said. To clarify, I surmised that all things being equal, the 35% more mass of a .308 BCG is going to put more compressive force on the buffer bumper than a 5.56 BCG, which then causes the buffer to subsequently "bounce" off the buffer tube and return the carrier to battery at a higher velocity than a 5.56 BCG would, which I suspect could lead to function issues not seen by a 5.56 at a similar carrier velocity.

But the reciprocating mass isn’t exposed to the energy or momentum of the cartridge, it’s exposed to the gas pressure at the port position (minus fitting losses in the gas system, naturally). The gas tubes are the same diameter, so the cross-sectional area is the same, and the gas port diameters recommended for both are similar (~.096” for 20” rifle length) - the only difference could be the pressure... but wait... 5.56x45 EPVAT standard is 62kpsi and change, while EPVAT for 7.62x51mm is 60kpsi and change. Looking at the pressure curve for each, and referencing the pressure at any gas port locations, you can see the resulting force on each action really isn’t so different: since Force = Pressure * Area and both pressures and areas are the same:

Again, you are inventing an argument I never made.

View attachment 830411

So you assume the 35% increased mass isn’t sufficient, but don’t look at any objective science to suggest WHY it’s insufficient - if the operating forces are so similar... seems like you just speculate a 2800ft.lb. rifle must operate much more violently than a 1300ft.lb. rifle, without understanding the actual forces involved.

Again, this is an argument I never made, and does not address the argument I did make.

 

[Varminterror]

Fair enough, I came at your issue from the front end, while you’re only thinking about the back end...

So now run your logic out....

F=ma...

Same force (described in my post above)... Increase mass by 35%, decrease acceleration by 35%. Decrease acceleration, decrease resultant velocity... so now the carrier isn’t hitting the extension as fast... V = V0 + A*T... we know V0 = 0 since the carrier started at rest, we don’t necessarily need to know A (acceleration) but we know it’s reduced by 35%. Time T is kinda tricky, since the time of exposure - the duration the gas tube is engaged with the key and the piston is under pressure, prior to clearing the vents - is dependent upon the displacement of the carrier, which is dependent upon its acceleration, which we know is reduced... so we expect T is increased, while A is decreased...

Solving for a descrete value takes more calculus than I care to do on a Sunday afternoon just to walk someone through an arbitrary thought experiment, but I think most folks are familiar with a paradigm - same force exerted on a larger mass yields a lower velocity. Hit a golf ball with a baseball bat, then hit a bowling ball with the same force, for example...

Ultimately, it’s not really a fair assumption to expect an AR-10 hammers the receiver extension any harder than an AR-15 by design. Overgassed or undermassed systems will be out of balance and exhibit excessive carrier speed, regardless of 10 or 15, but neither are designed to be overgassed or undermassed, nor the opposite.

 

[silicosys4]

Fair enough, I came at your issue from the front end, while you’re only thinking about the back end...

So now run your logic out....

F=ma...

Same force (described in my post above)... Increase mass by 35%, decrease acceleration by 35%. Decrease acceleration, decrease resultant velocity... so now the carrier isn’t hitting the extension as fast... V = V0 + A*T... we know V0 = 0 since the carrier started at rest, we don’t necessarily need to know A (acceleration) but we know it’s reduced by 35%. Time T is kinda tricky, since the time of exposure - the duration the gas tube is engaged with the key and the piston is under pressure, prior to clearing the vents - is dependent upon the displacement of the carrier, which is dependent upon its acceleration, which we know is reduced... so we expect T is increased, while A is decreased.

Again, these are arguments I never made.

...all things being equal

Solving for a descrete value takes more calculus than I care to do on a Sunday afternoon just to walk someone through an arbitrary thought experiment,

I'm not asking you to. You are inventing aspects to my question that are ultimately irrelevant to the discussion in order to redefine my argument.

...all things being equal

but I think most folks are familiar with a paradigm - same force exerted on a larger mass yields a lower velocity. Hit a golf ball with a baseball bat, then hit a bowling ball with the same force, for example..

Again, this is an argument I never made.

Ultimately, it’s not really a fair assumption to expect an AR-10 hammers the receiver extension any harder than an AR-15 by design. Overgassed or undermassed systems will be out of balance and exhibit excessive carrier speed, regardless of 10 or 15, but neither are designed to be overgassed or undermassed, nor the opposite.

Again, this is an argument I never made.

That's not at all what I said. To clarify, I surmised that all things being equal, the 35% more mass of a .308 BCG is going to put more compressive force on the buffer bumper than a 5.56 BCG, which then causes the buffer to subsequently "bounce" off the buffer tube and return the carrier to battery at a higher velocity than a 5.56 BCG would, which I suspect could lead to function issues not seen by a 5.56 at a similar carrier velocity.

We are talking about carriers at the same velocity and the effects thereof, not how they would get to that velocity.

 

[Varminterror]

all things being equal, the 35% more mass of a .308 BCG is going to put more compressive force on the buffer bumper than a 5.56 BCG

Ok.... You’re going to have to clarify why you believe this statement to be true.

I just elaborated on the fact the heavier carrier in the AR-10 is moving slower than that of an AR-15, “all things being equal” on the front end, proving there’s no need to assume any difference in how either buffer impacts the receiver extension at the rear. But you seem set on asserting the extra mass means it hits harder...

So why do you think it hits harder?

 

[silicosys4]

Ok.... You’re going to have to clarify why you believe this statement to be true.

I just elaborated on the fact the heavier carrier in the AR-10 is moving slower than that of an AR-15, “all things being equal” on the front end, proving there’s no need to assume any difference in how either buffer impacts the receiver extension at the rear. But you seem set on asserting the extra mass means it hits harder...

Actually, you are still redefining the context to better suit your argument. If you can't answer according to the parameters of my question, that's fine, neither can I, thats why its a question. There's no shame in admitting you don't know.

So why do you think it hits harder?

Math.
Force = Mass x Acceleration. A 17.9 Oz. 308 BCG is going to impact harder than an 11.6 Oz. 5.56 BCG at the same impact speed.

 

[Varminterror]

@silicosys4 - please take my statements here as an attempt to provide you with information to fill in the holes in your expectations. Putting real info into a speculation. No argument, just trying to provide information. But a lot of the basises for your assumptions just aren’t founded in science or reality.

It's my fault for presenting the information with the assumption you could finish the final step to understand the system on your own. I'm not changing your argument at all, I'm simply showing you how you have put bad information into your front end, so you're getting bad information out on the back end. I tried to explicate that your expectation of similar carrier speeds and "compressive forces" were incorrect, but I was trying to be considerate and not say that so directly, but I obviously riled you up enough that you're focused on the "argument" you feel we're having, instead of taking some free education to correct your incorrect assumptions.

So... Let's look EXACTLY at your statements, and see if we can fill in some gaps...

To clarify, I surmised that all things being equal, the 35% more mass of a .308 BCG is going to put more compressive force on the buffer bumper than a 5.56 BCG

The problem is the fact you're assuming the same carrier speed, neglecting the fact they are NOT traveling at the same speed, so when you apply "Force = Mass x Accleration," you don't get the outcome you're expecting simply because of the greater mass in the AR-10.

At its core, this 35% more mass exerting more compressive force on the buffer bumper" should obviously not make sense - you're talking about the same force going into the front of the system, and a greater force coming out on the backend for the system with the HEAVIER mass (even disregarding the lost velocity due to the buffer spring compression). This is why I brought up the bowling ball vs. the baseball - the same force on the front end yields a far lesser velocity in the heavier case. It was my fault for assuming you would infer the subsequent influence of that lesser velocity upon the "compressive force." So let's explore that...

F1 = m1 *a1, I described the math above which explicates the force at the front end of both the AR-15 and AR-10 are very similar, it also described that the resultant velocity on the carrier is LESS for the heavier AR-10 carrier than the AR-15. Right now, we're talking about a pretty similar net momentum, since they were both exposed to the same force on the front end... one light and fast, one heavy and slow...

So let's talk about what happens at the back end when the reciprocating mass impacts the receiver extension and exerts that "compressive force" on the buffer bumper. F2 = M1 * A2. The masses haven't changed - the carriers don't get any heavier or lighter during travel. Acceleration - or deceleration in this case - is the change in velocity over time (A = dV/dT), the time here also being dependent upon the velocity, since we're talking about landing on a spring (the rubberized buffer), so the lower the velocity, the longer the deceleration time - So the velocity is lower in the numerator, AND the time impulse is larger in the denominator, so the deceleration of the AR-10 buffer is less than that of the AR-15. Alternatively, the AR-15 carrier is lighter and moving faster, such it will have greater deceleration during impact A = dV/dT, where dV is larger - higher carrier speed going to zero, and T is shorter since the compression happens faster due to the higher velocity.

So the deceleration of the AR-15 carrier is greater than that of the AR-10 carrier. F = MA, So you have a heavier mass with a smaller deceleration in the AR-10, and a lighter mass with MORE deceleration in the AR-15. We're trading nickels here - there's no reason to think there's a greater "compressive force" in the AR-10, just because the mass is greater - it's moving slower too...

which then causes the buffer to subsequently "bounce" off the buffer tube and return the carrier to battery at a higher velocity than a 5.56 BCG would

So the discussion above proves the front half of the "bounce," the "compressive force" you mentioned. Let's now look at the back end of the bounce - because your statement that the AR-10 "carrier returns to battery at a higher velocity than a 5.56 BCG would," is backwards.

The discussion above proves your assumption about the AR-10 having a greater compressive force is incorrect. The two are roughly similar. So you're starting with F10 = F15, and again, F=MA... So we have the same force again working on two masses, one heavier, one lighter. The lighter mass, the AR-15 carrier, will be accelerated more than the heavier mass, so the AR-15 carrier will actually be the one with the higher velocity.

The best part here - even including the force of the buffer spring into these equations, the result is the same - Equal forces exist in both designs, and F=MA, so the heavier mass will be accelerated less, meaning lower carrier speed.

which I suspect could lead to function issues not seen by a 5.56 at a similar carrier velocity.

Just putting the capstone on this, because this is really the root of your confusion - they aren't at the same carrier velocity. The AR-15 carrier runs faster than the AR-10, so your assumption that the AR-10 carrier is flying backwards is incorrect, your assumption that the "compressive force" of the heavier AR-10 reciprocating mass impinging upon the buffer bumper is greater than an AR-15 is incorrect, and your assumption that the AR-10 carrier is flying back into battery at higher speed than in an AR-15 is incorrect.

I hope you don't take any of this as insult, as that's not my intent, but I tried to gently give you this information twice. Now you have it, plainly in front of you. Holding wrong information as truth either happens by lack of knowledge (not having the information), misunderstanding (incorrectly understanding the information), or denial (refusal to accept the information). I hope you've learned enough here to change your incorrect assumption about the difference in operating forces in the AR-10 vs. AR-15 - but if you don't, then it's really only denial.

 

[Varminterror]

Math.
Force = Mass x Acceleration. A 17.9 Oz. 308 BCG is going to impact harder than an 11.6 Oz. 5.56 BCG at the same impact speed.

You edited your post and added these details.

Yes, without question, if both were at the same speed, the heavier mass would hit harder.

However, as I stated in my last, your assumption that the two are traveling at the same speed is incorrect. So again, garbage in, garbage out. You put a bad assumption into you logic, and it yielded an incorrect result. The heavier 17.9oz 308 BCG isn't traveling at the same speed as the 11.6oz 5.56 BCG.

ETA:
I don't know how I missed this the first time. As I stated above - this has been the problem in your logic throughout the conversation:

We are talking about carriers at the same velocity and the effects thereof, not how they would get to that velocity.

We're NOT talking about carriers at the same Velocity. THAT was what I was trying to help you understand through all of these posts.

 

[silicosys4]

@silicosys4 - please take my statements here as an attempt to provide you with information to fill in the holes in your expectations. Putting real info into a speculation. No argument, just trying to provide information. But a lot of the basises for your assumptions just aren’t founded in science or reality.

It's my fault for presenting the information with the assumption you could finish the final step to understand the system on your own. I'm not changing your argument at all, I'm simply showing you how you have put bad information into your front end, so you're getting bad information out on the back end. I tried to explicate that your expectation of similar carrier speeds and "compressive forces" were incorrect, but I was trying to be considerate and not say that so directly, but I obviously riled you up enough that you're focused on the "argument" you feel we're having, instead of taking some free education to correct your incorrect assumptions.

I'm not riled up at all, we just have differing opinions and are discussing their merits, a situation that can be called a polite argument. No need for chest thumping , condescension, or hostilities.

I'm simply showing you how you have put bad information into your front end, so you're getting bad information out on the back end.

Well, like I said, I'm not entirely convinced. I've seen you redefine the parameters of the question in order to redefine it in a way you feel more comfortable addressing, and asserted that my question has no merit because its parameters are impossible to achieve, but I disagree.

So... Let's look EXACTLY at your statements, and see if we can fill in some gaps...
The problem is the fact you're assuming the same carrier speed, neglecting the fact they are NOT traveling at the same speed,

This is where we diverge. You simply state this is true and provided some chamber pressure data, data that I have an issue with.
First off, The following statement is cherry picking data to support your argument.

5.56x45 EPVAT standard is 62kpsi and change, while EPVAT for 7.62x51mm is 60kpsi and change.

Accepted SAAMI pressure limits for the .308 is 62,000psi and 55,000psi for the .223. There is a lot of ammo sold and fired in the U.S. that is loaded to SAAMI pressures, not NATO EPVAT pressures. Just as a guess, I'd say that the majority of ammo fired in the U.S. is going to be commercially loaded to SAAMI specs. Using EPVAT standards to support your argument that chamber pressures are going to be the same between the two in all cases is inappropriate, imo. There is a large amount of .308 ammunition fired through civilian owned rifles in the U.S. is going to be loaded to SAAMI specs, and a large amount of .223.

so when you apply "Force = Mass x Accleration," you don't get the outcome you're expecting simply because of the greater mass in the AR-10.

Sure, if you pad the numbers by using EPVAT standards and ignore the fact that SAAMI max chamber pressures are 13% higher in the .308 than the .223. Also, going by chamber pressure alone doesn't take into account the greater volume of gas produced by the .308 cartridge, nor does it account for different barrel lengths and different gas system lengths, different buffer weights, and different buffer spring rates, all of which have an influence on carrier velocity.

At its core, this 35% more mass exerting more compressive force on the buffer bumper" should obviously not make sense - you're talking about the same force going into the front of the system, and a greater force coming out on the backend for the system with the HEAVIER mass (even disregarding the lost velocity due to the buffer spring compression). This is why I brought up the bowling ball vs. the baseball - the same force on the front end yields a far lesser velocity in the heavier case. It was my fault for assuming you would infer the subsequent influence of that lesser velocity upon the "compressive force." So let's explore that...

This only makes sense if you assume that an overgassed .308 is incapable of seeing the same carrier velocity as a 5.56 carrier. I still have not seen sufficient supporting evidence to accept that premise.

F1 = m1 *a1, I described the math above which explicates the force at the front end of both the AR-15 and AR-10 are very similar, it also described that the resultant velocity on the carrier is LESS for the heavier AR-10 carrier than the AR-15. Right now, we're talking about a pretty similar net momentum, since they were both exposed to the same force on the front end... one light and fast, one heavy and slow...

So for the sake of argument lets assume you are correct. Lets assume the AR 308 carrier speed is 10% slower, even 20% slower. It is still 35% heavier than a 5.56 BCG, so even at a reduced speed relative to the 5.56 BCG it is still producing more impact force against the buffer tube. Math is important here, at some point the carrier velocity of the .308 BCG would be low enough relative to the carrier speed of a 5.56 BCG that it would have less impact energy, but we don't know where that number is and we don't know if the AR 308 BCG ever exceeds that velocity or not.

So let's talk about what happens at the back end when the reciprocating mass impacts the receiver extension and exerts that "compressive force" on the buffer bumper. F2 = M1 * A2. The masses haven't changed - the carriers don't get any heavier or lighter during travel. Acceleration - or deceleration in this case - is the change in velocity over time (A = dV/dT), the time here also being dependent upon the velocity, since we're talking about landing on a spring (the rubberized buffer), so the lower the velocity, the longer the deceleration time - So the velocity is lower in the numerator, AND the time impulse is larger in the denominator, so the deceleration of the AR-10 buffer is less than that of the AR-15. Alternatively, the AR-15 carrier is lighter and moving faster, such it will have greater deceleration during impact A = dV/dT, where dV is larger - higher carrier speed going to zero, and T is shorter since the compression happens faster due to the higher velocity.

So the deceleration of the AR-15 carrier is greater than that of the AR-10 carrier. F = MA, So you have a heavier mass with a smaller deceleration in the AR-10, and a lighter mass with MORE deceleration in the AR-15. We're trading nickels here - there's no reason to think there's a greater "compressive force" in the AR-10, just because the mass is greater - it's moving slower too...

Again, I haven't seen any supporting evidence to suggest that a SAAMI-spec loaded cartridge that produces 13% more chamber pressure and a significantly greater amount of gas volume is not capable of overcoming the 35% greater weight of a AR 308 to produce carrier impact energy greater than a 5.56 BCG.

So the discussion above proves the front half of the "bounce," the "compressive force" you mentioned. Let's now look at the back end of the bounce - because your statement that the AR-10 "carrier returns to battery at a higher velocity than a 5.56 BCG would," is backwards.

The discussion above proves your assumption about the AR-10 having a greater compressive force is incorrect. The two are roughly similar. So you're starting with F10 = F15, and again, F=MA... So we have the same force again working on two masses, one heavier, one lighter. The lighter mass, the AR-15 carrier, will be accelerated more than the heavier mass, so the AR-15 carrier will actually be the one with the higher velocity.

The best part here - even including the force of the buffer spring into these equations, the result is the same - Equal forces exist in both designs, and F=MA, so the heavier mass will be accelerated less, meaning lower carrier speed.

Again, this is all based on a premise that you have failed to prove sufficiently to convince me due to lack of evidence.

Just putting the capstone on this, because this is really the root of your confusion - they aren't at the same carrier velocity.

The AR-15 carrier runs faster than the AR-10, so your assumption that the AR-10 carrier is flying backwards is incorrect, your assumption that the "compressive force" of the heavier AR-10 reciprocating mass impinging upon the buffer bumper is greater than an AR-15 is incorrect, and your assumption that the AR-10 carrier is flying back into battery at higher speed than in an AR-15 is incorrect.

Again, as far as I can see it, your entire evidence to support this is NATO's EPVAT standards, which don't really apply to the real world as experienced by civilians in the U.S. firing commercially loaded ammo. You haven't addressed the much greater gas volume of the .308 cartridge when fired as compared to the .223, so I can't just accept your statement about carrier speed and impact energy to be true.

I hope you don't take any of this as insult, as that's not my intent, but I tried to gently give you this information twice. Now you have it, plainly in front of you. Holding wrong information as truth either happens by lack of knowledge (not having the information), misunderstanding (incorrectly understanding the information), or denial (refusal to accept the information). I hope you've learned enough here to change your incorrect assumption about the difference in operating forces in the AR-10 vs. AR-15 - but if you don't, then it's really only denial.

Oh, no insult taken, and I appreciate that you've taken a substantial amount of time with your explanations, but I'm afraid your explanations insufficiently address my question for my mind to have been changed. We'll just have to agree to disagree. Again, thank you for the information.

 

[Varminterror]

Accepted SAAMI pressure limits for the .308 is 62,000psi and 55,000psi for the .223.

This one's ridiculously easy, and I wish you were trying harder here.

Neither the AR-15 nor AR-10 were designed with SAAMI standards as their design basis. Both were designed to handle the same pressures.

So now you're cherry picking, running one design at its design point and one with reduced loads...

 

[silicosys4]

This one's ridiculously easy, and I wish you were trying harder here.

Neither the AR-15 nor AR-10 were designed with SAAMI standards as their design basis. Both were designed to handle the same pressures.

So now you're cherry picking, running one design at its design point and one with reduced loads...

Right..because no modern AR rifle differs in any way from the original design
Gotcha.
SAAMI spec ammo is the norm here, and SAAMI spec .308 is 13% higher SAAMI max pressure than .223, therefore the majority of .308 fired in the U.S. is going to have higher chamber pressure than the .223. Arguing that the rifles weren't designed around the ammo that will actually be fired through them in the real world as if that magically turns commercially loaded .308 and .223 into the same pressure cartridges...well that seems odd to me. Perhaps I'm misunderstanding you.

Sorry, but I don't see that you have shown any actual evidence to support your statements that AR 308 carrier speeds are insufficient to generate more impact energy at the buffer tube than a 5.56 BCG. You have made some correlations that do not indicate causation, but thats about it.

I think I've learned about all there is to learn here, so I think I'm done.
Thank you for the conversation

 

[Varminterror]

You haven't addressed the much greater gas volume of the .308 cartridge when fired as compared to the .223

Sure - the 308win has 74% greater case capacity. It also has 89% greater bore cross-sectional area, and only runs about 60% greater powder charge... So your argument here falls on my side - the 308win is proportionately less powerful for its bore capacity than the 5.56.

But really, pointing out the case capacity at all is just a red herring, since case capacity is wholly irrelevant in the conversation - we're not talking about blowback mechanisms, we're talking about gas operated pistons. Again, the port pressure is the important comparison - to which I'm sure you'd rebut, "but the larger case can flow more!" So then I'll point out the gas port is the same size, letting down pressure the same way, and the gas tube is the same diameter, transferring the same flow under the same pressure in either rifle, and THEN ending in the same sized gas key, with the same 5/16" travel prior to venting... But wait - that same pressure is being let down into a piston bore ~1/2" in an AR-15, and let down into a much larger piston bore of ~2/3" in an AR-10, and if we recall how gas pressure works - P1 * V1 = P2 * V2, we're starting at the same pressure and volume fed to the piston, but one bore is larger, so the net pressure in the AR-10 piston is lower. Moot point, of course, because it's proportionate to the area, so the force produced by the pressure in the piston is the same for both - one is lower pressure, but larger area, while the AR-15 is higher pressure in the piston, with a lower piston head area.

Here's a freebie though. Note the port pressure at 12" for this SAAMI Compliant 55kpsi 223rem load and compare it to the 12" port pressure in the graphic I shared earlier. Despite peaking at "13% lower chamber pressure," they both run just a shade under 20kpsi at the port - so again, the force on the reciprocating mass is the same for the AR-15 as it is in the AR-10, and all of the math I described above remains to apply - the AR-10 carrier runs slower than the AR-15, "all things being equal." Your assumption remains to be flawed

 

[Varminterror]

Sorry, but I don't see that you have shown any actual evidence to support your statements that AR 308 carrier speeds are insufficient to generate more impact energy at the buffer tube than a 5.56 BCG. You have made some correlations that do not indicate causation, but thats about it.

I've provided the mathematical and scientific pathway to reach a valid conclusion. You've provided nothing to support your claims that the heavier AR-1o reciprocating mass bounces harder off of the receiver extension, while I'm showing proven science (physics) and data to support the port pressure equivalencies and resulting relative velocities of the reciprocating mass. Your assumption was wrong, that's fine. You looked at a lot of logical parameters, followed a bad path, and landed at a bad conclusion - that's fine. But the data is there. Both systems run roughly 20kpsi at the port, even when comparing 62kpsi 308win SAAMI loads to 55kpsi 223rem loads. The port pressure is roughly the same, the port size the same, tube length and diameter the same, key the same... the Carriers are exposed to the same force, one is heavier, so it is accelerated less, and therefore, your assertion the AR-10 carrier speed is greater is wrong.

But if you could and would provide ANY DATA OR SCIENCE WHATSOEVER which actually refutes the data and science I have provided here and supports your hypothesis that the AR-10 port pressure or carrier speed is greater than that of the AR-15 and that resultingly, the AR-10's reciprocating mass exerts a greater "compressive force" on the buffer bumper, I'd be happy to learn something new today.

 

[silicosys4]

]

I've provided the mathematical and scientific pathway to reach a valid conclusion. You've provided nothing to support your claims that the heavier AR-1o reciprocating mass bounces harder off of the receiver extension, while I'm showing proven science (physics) and data to support the port pressure equivalencies and resulting relative velocities of the reciprocating mass.

So if you could and would provide ANY DATA OR SCIENCE WHATSOEVER which actually refutes the data and science I have provided here and supports your hypothesis that the AR-10 carrier speed is greater than that of the AR-15 and that the AR-10's reciprocating mass exerts a greater "compressive force" on the buffer bumper, I'd be happy to learn something new today.

Lets just get down to the nitty gritty. If you can't answer the following, there is really no reason to continue the conversation, as its just conjecture on both sides.

What is the maximum velocity of a 17.9oz. AR 308 carrier?
What is the maximum velocity of an 11.6oz. 5.56 carrier?

Simple. Direct.
After you give those numbers, the rest is just simple math. m/sec, in/sec, cm/sec, yards/hour, however you want to give that info, I'm all ears.

 

[Varminterror]

@silcosys4

How about you provide ANYTHING which says it could even be possible. Because I've provided sufficient evidence to show you're pointing the wrong direction. You're the one stating AR-10's batter the action worse than AR-15's, and claiming the increased carrier speed, despite science and data to refute your faulty hypothesis.

I'll admit, I'd have to do a lot of digging to solve the system, and make a lot of assumptions which I'm assured you would use as an excuse to negate the entire exercise. I don't know the fitting losses of the gas block and would have to make some assumptions to apply empirical nomograms, and work through rough let down ratios. It can be done, but it'll be no more or less valid that the data and science I've provided you above. Frankly, you're argumentative and not worth my time to even go through that exercise. The math is all in front of you.

I provided the pressure data based on the design parameters - you moved the target a bit with a valid argument - using SAAMI data does make sense. So I provided SAAMI compliant data for comparison. It didn't change the outcome, so now you're taking your toys and going home because you're in a corner and can't admit you were wrong.

Alternatively, you have not provided anything at all to support the assertion the AR-10 carrier speed is greater than the AR-15. While I've provided pressure curves which support the forces acting on the carriers are the same - which you seem to refuse is possible, and have provided sound science to show the forces exerted on the carriers are equivalent, and the science to illustrate the resulting velocity for the AR-10 will be less than that of the AR-15.

You don't have to like it, and don't have to like how I conveyed it. I've tried to be civil, but you're not supporting your argument at all, and refusing to accept good science. Remember what I said above about denial? It ain't just a river in Egypt, my friend....

It's unfortunate, I'm sure this will be locked sooner than later, but the discussion of these operating systems is good information for folks to have, lest they be fooled into bad assumptions about their rifles, and worse, further spread bad info.

 

[silicosys4]

@silcosys4

How about you provide ANYTHING which says it could even be possible. Because I've provided sufficient evidence to show you're pointing the wrong direction. You're the one stating AR-10's batter the action worse than AR-15's, and claiming the increased carrier speed, despite science and data to refute your faulty hypothesis.

I'll admit, I'd have to do a lot of digging to solve the system, and make a lot of assumptions which I'm assured you would use as an excuse to negate the entire exercise. I don't know the fitting losses of the gas block and would have to make some assumptions to apply empirical nomograms, and work through rough let down ratios. It can be done, but it'll be no more or less valid that the data and science I've provided you above. Frankly, you're argumentative and not worth my time to even go through that exercise. The math is all in front of you.

I provided the pressure data based on the design parameters - you moved the target a bit with a valid argument - using SAAMI data does make sense. So I provided SAAMI compliant data for comparison. It didn't change the outcome, so now you're taking your toys and going home because you're in a corner and can't admit you were wrong.

Alternatively, you have not provided anything at all to support the assertion the AR-10 carrier speed is greater than the AR-15. While I've provided pressure curves which support the forces acting on the carriers are the same - which you seem to refuse is possible, and have provided sound science to show the forces exerted on the carriers are equivalent, and the science to illustrate the resulting velocity for the AR-10 will be less than that of the AR-15.

You don't have to like it, and don't have to like how I conveyed it. I've tried to be civil, but you're not supporting your argument at all, and refusing to accept good science. Remember what I said above about denial? It ain't just a river in Egypt, my friend....

It's unfortunate, I'm sure this will be locked sooner than later, but the discussion of these operating systems is good information for folks to have, lest they be fooled into bad assumptions about their rifles, and worse, further spread bad info.

So I'll take that as a no, you don't know the maximum carrier velocities of either the AR 308 BCG or the 5.56 BCG.
Thanks

 

[Varminterror]

So I'll take that as a no, you don't know the maximum carrier velocities of either the AR 308 BCG or the 5.56 BCG.
Thanks

I don't know them off of the cuff. I'll expect you have none such data in your pocket either, nor understanding of how to get it, else you'd have provided it by now and saved us both the time and trouble.

What's your beef with the provided pressure curves now? You haven't done anything but punt yourself away from the discussion since I provided it. You asked for a comparison of SAAMI compliant data, and I provided it. Then you bowed out. Should I take that as your concession of defeat? I provided evidence to support a 60kpsi load and a 55kpsi load have similar port pressures - but you don't want to accept it. What about this data doesn't jive to you? I didn't generate either curve, I simply googled the pressure curves and made sure the load data and bullet weights made sense for the cartridge (i.e. not talking about a 110grn bullet in 308, or a 90grn pill in 223). The data is in front of you, but you don't agree with it. So what do you find to be incorrrect in that pressure data?

You also argued that the larger gas volume of the 308win was relevant - to which I replied with the relative case volumes, charge weights, and bore areas, you didn't give any response to that data, so where do you fall in terms of your assertion the importance of the gas volume, given this new data? When you presented that statement - the 308win has a greater gas volume - I expected more follow up to support the argument that greater gas volume would result in increased carrier speed, but we didn't get any further discussion there. Were you just throwing spaghetti at the wall to see what might stick, or do you have something to support that the larger gas volume increases the carrier speed?

We're obviously disconnected here, and understanding what particular bit of the data I provided above just doesn't make sense.

I was taught this type of science around the AR design over 20yrs ago, this isn't anything new. I'd love to find something new which overturned my understanding of these systems. We haven't even started to talk about fully automatic fire and reciprocating mass affect on cyclic rate - there are 5.56 rifles out there running 600rpm and others running 1200rpm. I've not seen reports on a large format in excess of 750, but I expect that's more about designed controllability - and yes, it's all controlled by port pressure (diameter and position) and reciprocating mass. Wanna slow down your BCG? You add weight. In the case of the AR-15 vs. AR-10, that weight increase is inherent to the design, so I'm really failing to understand why you think the result would be different.

 

[silicosys4]

Thats a lot of writing. I see no carrier velocity numbers though.

 

[Matt.Cross]

@Varminterror

Thanks for your contributions to this thread. I think anyone with any critical thinking skills will be enriched by examining the information you have shared here, and will also ultimately find it to be true. It's an unfortunate fact of life at this period in time that a reply is often interpreted as an answer when a given topic is discussed or debated.