Is a mid length gas system really better in a 16" AR?

[Malamute]

Interesting, but bummer, no info on which makers use which size ports.

 

[Varminterror]

If making the port size of an M4 deliver the same pressure to the carrier as a 20" rifle, it should offer the same function, but theres something more to it than that. They didn't do that for some reason.

When the M4 was revamped, it was NOT attempting to replicate the same pressure as the 20" rifle. Again, this comparison is moot. You'll note, the carbine has a greater rate of fire than the rifle (everywhere except Wikipedia, of course), so it's recognized the bolt speed SHOULD be greater in the carbines.

Guys run carbine length systems with lightweight BCG's and carbine buffers in race rifles, and have been for many years. If it's over gassed, they either tune their loads, spring rate, and/or gas flow. Running rifles hard wears them out, arguing about port erosion at 7.8" vs. 9.7" is moot - many different combinations can work, and that's even before you step into different cartridges within the platform. It's pointless to argue that ALL AR platform rifles should run exactly the same pressure, spring rate, dwell time, cyclic rate, carrier weight, buffer weight, etc etc as the original 20" rifle - a semiautomatic civilian rifle has very different operating conditions from a select fire military arm, and even within the models employed by the military, you'll find VERY different applications, from a high rate short barrel submachine gun XM177 to a long barreled semiauto only SAM-R or Mk 12.

There's no reason to say a civilian AR-15 in ANY form needs to have the same design parameters for spring rate, gas flow, dwell time, carrier weight, buffer weight, or rate of fire as an M16A2, nor an M4, and mechanically trying to restrict them to that point is just silly.

I also NEVER said I "think military rifles with adjustable gas systems don't work through restriction."

 

[Varminterror]

BTW, here's a list of gas port sizes used in different AR set ups. You can see that 16" carbines currently use the smallest port size:

And doesn't that make sense to you? Carbine length gas in a 14.5" and a 16" will have the same pressure, but the 16" will have a longer gas impulse (greater dwell time), so it has the luxury of being able to choke off a little flow and still cycle reliably.

 

[RX-79G]

And doesn't that make sense to you? Carbine length gas in a 14.5" and a 16" will have the same pressure, but the 16" will have a longer gas impulse (greater dwell time), so it has the luxury of being able to choke off a little flow and still cycle reliably.

No, it doesn't surprise me. It would surprise me if someone had a business restricting the gas ports even more than the small port size already in use for 16" carbines, considering erosion and the already small size.

I also NEVER said I "think military rifles with adjustable gas systems don't work through restriction."

No, you didn't. You misunderstood my question, so I will re-word it:

Given your knowledge of gas systems, could you give your opinion why military adjustable gas systems use venting rather than restriction?

 

[Varminterror]

Bringing up vented piston system firearms in a thread about DI AR's is a pretty desperate grab. The operating principle is different, so the design principles are different.

Venting in piston systems are designed to purge gas pressure after the piston has already began its travel and the bolt and piston mass have sufficient velocity (resultant momentum). These vents are purge vents, the excessive flow and pressure relieved in these ports effectively allow the gas system to be over designed for flow, without battering the action. They get all of the bolt velocity they need, but never more (within reasonable limits).

DI systems - as in the AR - don't have this luxury, as they don't have a piston to act as a mechanical "valve" ahead of the purge ports.

You might be surprised, some like the M240 and 249 do operate by restriction - as the gas regulator collars (incorrectly named as such in their manuals) are adjustable effectively by port diameter.

 

[Malamute]

When the M4 was revamped, it was NOT attempting to replicate the same pressure as the 20" rifle. Again, this comparison is moot. You'll note, the carbine has a greater rate of fire than the rifle (everywhere except Wikipedia, of course), so it's recognized the bolt speed SHOULD be greater in the carbines.

Guys run carbine length systems with lightweight BCG's and carbine buffers in race rifles, and have been for many years. If it's over gassed, they either tune their loads, spring rate, and/or gas flow. Running rifles hard wears them out, arguing about port erosion at 7.8" vs. 9.7" is moot - many different combinations can work, and that's even before you step into different cartridges within the platform. It's pointless to argue that ALL AR platform rifles should run exactly the same pressure, spring rate, dwell time, cyclic rate, carrier weight, buffer weight, etc etc as the original 20" rifle - a semiautomatic civilian rifle has very different operating conditions from a select fire military arm, and even within the models employed by the military, you'll find VERY different applications, from a high rate short barrel submachine gun XM177 to a long barreled semiauto only SAM-R or Mk 12.

There's no reason to say a civilian AR-15 in ANY form needs to have the same design parameters for spring rate, gas flow, dwell time, carrier weight, buffer weight, or rate of fire as an M16A2, nor an M4, and mechanically trying to restrict them to that point is just silly.

I also NEVER said I "think military rifles with adjustable gas systems don't work through restriction."

I may have worded my previous post poorly, because none of your post seemed to address what I was trying to ask. I'm familiar with most of what you stated in the above quoted post.

I was not proposing making M4 gas the same as a 20" gun. You stated that you used adjustable gas blocks for customers. You stated that I was incorrect in believing that gas pressure was the same even if more volume restricted with a smaller orifice or adjustable gas block. I was asking why the port pressure wasn't reduced to a lower level in the military M4s if that's an effective procedure that doesn't compromise reliability, and implied theres likely more to it than simply making a smaller orifice/adjustable gas block/whatever and lowering pressure in M4s. I believe M4s operate at the pressure level they do for a good reason and tinkering with adjustable gas blocks aren't going to improve overall reliability. Increased port pressure and carrier velocity etc is just part of the tradeoff for having a gun that short, or it would have been changed in its development. They work as they are. The sum of their parts and dimensions is part of why they work, even with the presumably less desireable higher pressure and carrier velocity.

The only real reason I can think of to duplicate the operating pressures/carrier velocities etc on civilian arms is to duplicate the reliability of the known quantity military arms, and not to be a beta tester of an experimental and theoretical improvement. I'm happy with the same basic function in proven military designs, with the exception of being willing to use a mid, even when it doesn't have the same level of proven data record of reliability of the carbine system. My use of ARs is more recreational than professional though. If my life totally relied on my gun working in all conditions, and my replacement parts were issued from military supplies, Id choose a totally proven design rather than a civvy design without the same level of time and field combat proven performance. Id likely also choose a 20" fixed stock gun, as I like them more in nearly every way and prefer the lower muzzle blast level.

 

[Varminterror]

I was asking why the port pressure wasn't reduced to a lower level in the military M4s

In my personal opinion(s), 1) because military M4's are designed to run a higher cyclic rate than needed for civilian application. They ship with H2 or H3 buffers to provide enough inertia for peak pressure resistance (staying locked long enough), then want enough pressure to blast a fast cycle speed. THIS is what I consider to be "the reason," but there are other considerations tied with it which support why it works for them, and isn't necessarily applicable to civilian application. 2) Military weapons aren't designed to last forever, so they can be cycled at high rate to serve their mission objective, then get scrapped at a lower round count than most civilians would consider the "life" of their firearm. 3) M4's don't need to worry about reloading ammunition, so they can be hateful and abusive to brass without concern. I might not want my cases to come out of my 3gun rifle with bulges in front of the web or flattened case necks, but a soldier running an M4 overseas isn't likely going to police his brass after an engagement the way I do after a match. Soldiers don't even really have to care if they get split cases, as long as the cases get the heck out of Dodge after they do their job and the next one feeds in reliably and the bullet hits what they want, the M4 can be very hateful to its brass.

Tying into the other thought process of military machine guns - for example - the M240 has an adjustable gas regulator to allow 3 different cyclic rates. In different applications, different cycle rates are desired.

 

[Malamute]

All good info and thoughts, thanks!

I think you are correct about the general reason for the acceptance of harsh operating pressures and speeds in the M4.

 

[MistWolf]

There is a lot of conjecture being thrown around in this thread with very little understanding of internal ballistics, pressure curves, or basic physics.

It's not a complicated system...

...The engineering work to determine appropriate port size and pressure drop across the gas tube, the transient state operation of the opening bolt and declining pressure over dwell time is well within the reach of anyone who finished their freshman year of engineering school...

...engineers today have tools on hand which can do engineering design computation within minutes and seconds which would have taken dozens if not hundreds of hours of hand calculations...

This. We don't need to shoot middies to destruction over and over again to prove they have an advantage. We already have two proven data points- the 20" rifle and the 14.5" M4. The 16" middy falls in between these two and now it affects the reliability and durability isn't a theory. It can be extrapolated. Saying the middy is nothing more than theory because it's not as proven is like saying only in theory does a 17.35" barrel gives higher velocity than a 14.5" barrel because we've never tested a barrel in that length. In fact, we have enough data points to extrapolate data that, in this example, proves the longer barrel does indeed produce greater velocity

 

[Malamute]

This. We don't need to shoot middies to destruction over and over again to prove they have an advantage. We already have two proven data points- the 20" rifle and the 14.5" M4. The 16" middy falls in between these two and now it affects the reliability and durability isn't a theory. It can be extrapolated. Saying the middy is nothing more than theory because it's not as proven is like saying only in theory does a 17.35" barrel gives higher velocity than a 14.5" barrel because we've never tested a barrel in that length. In fact, we have enough data points to extrapolate data that, in this example, proves the longer barrel does indeed produce greater velocity

I believe the bold part is a contradiction. Extrapolation isn't solid data, its a conclusion surmised from other information without direct information to prove the conclusion.

I think we do need to shoot mids to destruction with statistically significant example numbers to conclusively prove they make a difference, otherwise its guessing (or extrapolation) based on similar information. Otherwise, its a guess or estimation of how much difference it makes in longevity of parts. It makes sense that they should, it just hasn't been conclusively proved so far.

Extrapolation, to form an opinion or to make an estimate about something from known facts.

 

[MistWolf]

...Change the diameter of the gas port and all of the above conjecture about NFA compliance and the 16" barrel vs. 14.5" using the same gas system length becomes moot. A smaller port on a shorter Carbine system can actually expose the action to LESS pressure than a larger port on a Mid-length system...

This fact all too often gets overlooked.

The foundation of the system is the momentum of the reciprocating parts. Mass and velocity have to be balanced for reliable extraction and feeding without pounding the rifle to death or pushing the rifle around too much or bouncing out of battery or knocking the bullet loose from the case during chambering or breaking parts in the fire control group. The gas and spring rate have to be tuned to support the function reciprocating mass.

You can deviate from the norm. The AR operating system is actually very forgiving. But in doing so, you're trading something away, usually resulting in getting narrower parameters of reliable functioning

 

[MistWolf]

I believe the bold part is a contradiction. Extrapolation isn't solid data, its a conclusion surmised from other information without direct information to prove the conclusion.

I think we do need to shoot mids to destruction with statistically significant example numbers to conclusively prove they make a difference, otherwise its guessing (or extrapolation) based on similar information. Otherwise, its a guess or estimation of how much difference it makes in longevity of parts. It makes sense that they should, it just hasn't been conclusively proved so far.

Extrapolation, to form an opinion or to make an estimate about something from known facts.

We have enough to show that the performance of 16" middies falls between the 20" rifle and the 14.5 carbine.

We know that both proven systems are reliable and durable. Even if we had hard numbers to show where the middy falls between the two, it's still a matter of opinion as to whether or not those differences are great enough for anyone to make a definitive choice.

I've tried the AR with all three lengths of gas systems with different barrel lengths. What I learned is, getting the right reciprocating mass and speed is important and to do that, you've got to get the gas flow right

 

[Shawn Dodson]

In my experience a carbine length 16" bbl experiences greater muzzle jump. Adding a heavier buffer doesn't decrease muzzle jump - only an adjustable gas block does (with no need to change the buffer) to decrease gas pressure and volume.

It seems all AR barrels have oversize gas ports, the reason being to increase functional reliability with a broad range of .223 - 5.56 ammunition.

 

[RX-79G]

Bringing up vented piston system firearms in a thread about DI AR's is a pretty desperate grab. The operating principle is different, so the design principles are different.

Venting in piston systems are designed to purge gas pressure after the piston has already began its travel and the bolt and piston mass have sufficient velocity (resultant momentum). These vents are purge vents, the excessive flow and pressure relieved in these ports effectively allow the gas system to be over designed for flow, without battering the action. They get all of the bolt velocity they need, but never more (within reasonable limits).

DI systems - as in the AR - don't have this luxury, as they don't have a piston to act as a mechanical "valve" ahead of the purge ports.

You might be surprised, some like the M240 and 249 do operate by restriction - as the gas regulator collars are adjustable effectively by port diameter.

My M96 had a gas system that vented separately from the position of the gas piston. It vents immediately prior to piston movement.

The AR has exactly the same "luxury" - that's what those vents in the in the carrier do.

An AR gas system is a piston system, with the piston located behind the bolt.

 

[MistWolf]

In my experience a carbine length 16" bbl experiences greater muzzle jump. Adding a heavier buffer doesn't decrease muzzle jump - only an adjustable gas block does (with no need to change the buffer) to decrease gas pressure and volume.

It seems all AR barrels have oversize gas ports, the reason being to increase functional reliability with a broad range of .223 - 5.56 ammunition.

I got a 10.5" barrel that came with a .083" gas port. I started with a carbine weight buffer and carrier speed was definitely excessive. The carrier hit the hammer so hard, it was felt through the trigger as a sharp, painful sting. A heavier buffer slowed things down enough that the shorty was shootable, but it was still over gassed, so an adjustable gas block was installed.

I've got it adjusted so it runs smoothly with the correct buffer weight and locks back on the last round. Close the block one more click and it will eject reliably but not lock back on the last round. The adjustable gas block was the best thing I did for that shorty. Made all the difference in the world

 

[Varminterror]

He should have said INTERPOLATE instead of extrapolate. But his principle stands.

It doesn't take 50yrs to prove a firearm design is as good as an M16. plenty of rifles in almost every possible combination imaginable have been tested and proven - whether it's different length barrel and gas lengths, or even other cartrisges. Most shooters are unfamiliar with the idea of "wearing out" a firearm in their lifetime, let alone within a year, so it seems to them like "proving" a platform takes generations - frankly, that's not the case. Arms designers have proven metrics based on longevity studies of past firearms, and testing protocols to prove out new firearms to meet or exceed minimal acceptable standards.

Most shooters won't even burn out ONE barrel in their life, where as some others wear out a few barrels a year. Couple with that fire control groups, extractor springs, gas rings, barrel bushings, recoil springs, ejectors, operating rods, etc etc... Arms manufacturers can proof test a design within months.

PRODUCTION lines tend to be where a design fails, but the product design in itself is easily tested.

 

[Varminterror]

My M96 had a gas system that vented separately from the position of the gas piston. It vents immediately prior to piston movement.

The AR has exactly the same "luxury" - that's what those vents in the in the carrier do.

An AR gas system is a piston system, with the piston located behind the bolt.

Your understanding of both systems is flawed if you believe these systems are the same.

Your M96 bleeds gas off from the "work side" of the piston, whereas the AR design "bleeds" off of the non- work side. In other words, the M96 gas bleeding pressure relief is exactly that - an off-bleeding orifice type gas pressure relief, such the gas pressure is reduced before (and while) acting on the piston.

The AR is a completely different design. The force of the gas pressure reaching the vents has already acted upon the carrier and bolt before it ever reaches the relief vents. The M96 vents reduce the pressure to which the operating system is exposed, AR-15 vents in the carrier do not reduce the pressure to which it's exposed, but rather they reduce the duration of the pressure impulse. Much like other piston systems (SKS, AK, Garand/M1a, etc etc), the action parts of an AR have to travel a fixed distance before the bleed vents are exposed, which is NOT the case in the M96 adjustable block.

 

[RX-79G]

Your understanding of both systems is flawed if you believe these regulating systems are the same.

Your M96 bleeds gas off from the "work side" of the piston, whereas the AR design "bleeds" off of the non- work side. In other words, the M96 gas regulator is exactly that - an off-bleeding orifice type gas pressure regulator, such the gas pressure is reduced before (and while) acting on the piston.

The AR is a completely different design. The force of the gas pressure reaching the vents has already acted upon the carrier and bolt before it ever reaches the bleed vents. The M96 vents reduce the pressure to which the operating system is exposed, AR-15 vents in the carrier do not reduce the pressure to which it's exposed, but rather they reduce the duration of the pressure impulse. Much like other piston systems (SKS, AK, Garand/M1a, etc etc), the action parts of an AR have to travel a fixed distance before the bleed vents are exposed, which is NOT the case in the M96 regulator.

You misunderstood my reply.

I addressed pre-piston venting with the M96.

And I addressed your statement that the AR design doesn't have the luxury of FN FAL type venting. It does, since it also vents after the piston has moved, just like the FAL you compared it to. It isn't adjustable like the FAL, but it vents the same way.


Of course, you can also put a pre-piston vent directly in the carrier, as Gemtech and this 2A Armament did:

 

[Varminterror]

Gas vents located at different positions in the gas chamber serve different purposes.

Again - the FN type gas pressure regulating vents in the working end of the gas chamber do NOT serve in the same role as purge vents in other action types. Both reduce pressure, but one system is reducing force against the inertial resistance of the action before anything moves. The other purges force, effectively discontinuing the force impulse, after the force has already started acting.

It might seem like a vent is a vent if you don't understand mechanical system design, but from an engineering standpoint, the two are VERY dissimilar.

 

[RX-79G]

That's not how your statement reads:



So is it the exact same luxury? Or does it not have the same luxury? Can't be both - but you've now said both.

I'm sorry for the confusion. I addressed two separate parts of your post, and I had thought the use of your word - "luxury" - would clue you into which parts I was addressing. Here it is in full:

You claimed that adjust venting was post-piston movement, as on the FAL. I refuted your statement with an example of pre-piston movement venting with the M96. I also just provided another with the adjustable carriers.

You claimed that the AR gas system didn't have the luxury of venting after piston movement. I refuted this statement by pointing out that is exactly what the two vent holes in the carrier do - vent gas after the carrier has moved sufficiently.


So gas is vented prior to piston movement in several systems, and the AR DOES vent after the "piston" moves, just like a FAL. Which leads us back to my question - why do you think restricting gas flow is a good way to manage high pressures rather than venting?

 

[Varminterror]

Venting after movement does not reduce the maximum pressure the action is exposed to. It's like picking up a lighter hammer after you already hit someone in the face with an 8lb sledge hammer.

Venting BEFORE movement is like you never had access to a large hammer...

 

[Varminterror]

I don't see what you're trying to prove here. You're insisting upon some point by bringing up more and more designs in which orifice restrictions are used to purge pressure from one end or the other of a gas cycle, in an attempt to prove military rifles use "bleed" instead of restriction in a conversation about mid length vs carbine length AR-15's which don't use those designs...

I'll ask you this - do you understand how the pressure regulator (incorrectly dubbed a regulator by the manual) on your M96 gas block worked? It's nothing but different sized orifices - different sized restrictions. So even though it is "bleeding" gas, it is operating under the same principle - different size ports cause different pressure drops, or in other words, represent different levels of pressure restriction. So even your "bleed" systems are still restrictor based systems, just restricting what goes out, instead of restricting what goes in, as in the case of an adjustable gas block.

And of course, the concept killer for bleed systems - if I bleed pressure out of the gas chamber to limit my gas flow to my action instead of restricting gas flow into the chamber, I lose energy through the port which does nothing for me. If I use an adjustable gas block to restrict my gas flow into my gas chamber and into my action, none of that energy is lost. So in a bleed system, energy goes 3 places: action work, bullet work, and blasting into the atmosphere doing nothing. In a throttled system, my energy either does action work or bullet work, without the waste.

 

[RX-79G]

I don't see what you're trying to prove here. You're insisting upon some point by bringing up more and more designs in which orifice restrictions are used to purge pressure from one end or the other of a gas cycle, in an attempt to prove military rifles use "bleed" instead of restriction.

I'll ask you this - do you understand how the pressure regulator on your M96 gas block worked? It's nothing but different sized orifices - different sized restrictions. So even though it is "bleeding" gas, it is operating under the same principle - different size ports cause different pressure drops, or in other words, represent different levels of pressure restriction. So even your "bleed" systems are still restrictor based systems, just restricting what goes out, instead of restricting what goes in, as in the case of an adjustable gas block.

And of course, the concept killer for bleed systems - if I bleed pressure out of the gas chamber to limit my gas flow to my action instead of restricting gas flow into the chamber, I lose energy through the port which does nothing for me. If I use an adjustable gas block to restrict my gas flow into my gas chamber and into my action, none of that energy is lost. So in a bleed system, energy goes 3 places: action work, bullet work, and blasting into the atmosphere doing nothing. In a throttled system, my energy either does action work or bullet work, without the waste.

I'm puzzled by your answer. A vent isn't simultaneously a restricter. The M96 went from no vent to progressively larger vents. There is no point where the main gas flow is restricted to the point that it greatly increases in velocity and erosion.

The second thing I find puzzling is your answer that it is bad if energy is lost. It isn't a problem - it is the whole point. A carbine gas system puts too much energy into the bolt system, causing unnecessary recoil and cyclic rates. Gas venting gets rid of that unnecessary energy without resorting to the "nozzle problem" where you have a sacrificial orifice that is smaller than ideal so it wears fast.

The only energy loss that would matter would be if you could show that opening up the gas adjustment on a pre-piston actually had a serious effect on muzzle velocity. However, no gas bleed system is going to waste any more muzzle energy than a overpowered standard carbine gas system.

 

[Varminterror]

The vents on your M96 "restrict" the flow out of the vent. Smaller ports on the dial restrict it more. That's how they reduce pressure AND flow to the action - it's pretty simple science. The pressure they vent reduces the gas chamber pressure.

Here are some numbers as an example:

Say the powder charge in a 5.56 generates 200 units of work.

An M96 barrel port allows 15 units of work into the gas chamber. The action requires 10 units to operate. The bleed vents then must bleed off 5 units to the atmosphere. 185 push the bullet out of the muzzle, 10 cycle the action, 5 purge and do nothing.

Alternatively, an M4 with an adjustable gas block, also needing 10 units to cycle. So the gas block is throttled to only release those 10 units. Now 190 units are applied to the bullet, 10 to the action, and nothing is wasted - and I get ~3% more power out of my charge applied to my bullet.

And of course, since you bring up sacrificial nozzles - in an adjustable gas block system, the "nozzle" is generated between a 7 cent set screw and a $50-75 gas block, as the ultimate restrictive point is not within the barrel itself.

And of course beyond that - show me a barrel where port erosion, while terrible looking, has occurred independently of throat or bore erosion, causing a loss of accuracy. Barrels burn out, that's a reality. Erosion of the port is part of that, but it's not the ONLY part.

Keep digging - still doesn't change the fact throttling gas flow is regularly used in multiple firearms designs and bringing up the FN pre-cycle design of the M96 has absolutely nothing to do with the reliability conversation between a mid-length or carbine length AR carbine.

 

[MistWolf]

None of the systems used in self loading rifles regulate pressure. They regulate flow.

There isn't any practical difference between regulating flow through a bleed off valve, or restriction. Both work. In the case of the FAL, a larger gas port has to be used