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

[md7]

I'm not the best person to dive into and explain all of the science behind the designs, and provide meaningful input as it pertains to overall serviceability and lifespan of components between carbine vs mid length.

Like Azrocks, I do appreciate everyone's thoughts. It's been an interesting thread to follow.

For me, I'm simply content using quality carbine and/or mid length rifles. Both have good enough track records for me (ymmv) to feel good about using.

 

[UltraT]

The AR15 was designed to tap gas with a .096" hole and let it move through the .120" tube so the temp, pressure and flow would be as smooth as possible, preventing the particulate from condensing out of gas and fouling the system. If you have a carbine with a .0625 gas port, you've already restricted your gas path by nearly 60%. Add more restriction on the other side of that, and then open the gas path back up to .120" in the tube and you have created a hot flame that is then rapidly cooled, condensing the fouling onto the tube walls, as well as whatever material was burned out of the restriction.

That will work for quite awhile, but is pretty much the opposite of smooth, non-turbulent anti-fouling flow the 20" rifle was built to have.

I get that there would be more port erosion over time with more of a nozzle effect, but does more fouling in the gas tube actually occur from this in the real world? Has anyone actually removed their gas tube and said 'Wow there's a lot more fouling in here after I put in an adjustable gas block' ? From what I've seen, most people don't even need to clean they're gas tubes and they don't have problems.

 

[Malamute]

No... in that case I'd prefer the middy. If I can get reliable cycling with less wear & tear on the rifle, then that's the route I'm going to go. It doesn't matter to me if I can quantify the impact of the increase in heat/pressure. If I can get the same functional reliability and be kinder to the weapon, that makes the most sense to me. There's always the issue of premature failure to contend with... it isn't just a matter of component wear. Without an AGB, I see absolutely no advantage to a carbine, and the potential for a disadvantage. Since they'll both cost the same, I still think a mid-length makes more sense (in 16") in this case.

Does this line of thinking make sense, or am I missing something (sincere question)?

Have you worn out some carbines? Your post indicates you aren't happy with the lifespan of a carbine. Someone I know of said hed "worn out almost 10 carbines", going about 20,000 rounds each before he decided to retire them and get a fresh one. I believe he said there were no parts replacements, and all were Colt guns. His use was for work.

What is the type of premature failure you are concerned with?

We don't know for certain if the functional reliability is the same, theres never been any side by side scientifically done tests that anyone knows of, or even any anecdotal reports that have used large enough numbers of both to make concrete conclusions from them. If someone could even say they used 20 guns of known quality and the parts lasted X number of rounds, it would be more than we have at the moment.

Pat Rogers wrote that he had two bolt lugs break off the BCM mid at 16400 rds that he had in his armory and was the subject of the Filthy 14 article, along with assorted other small parts like extractors and extractor springs at various intervals. He felt that was very acceptable life of all those parts. Then we have the owner of Battlefield LV that said his Colt M4 bolts were running to 50,000 rounds. So who can really say what the improvement is? If there is any really difference, it will pale to insignificance in the price of the handful of parts compared to the cost of the ammo to actually wear one out. If you like mids, buy one. Justifying it with lifespan of parts isnt really provable yet. I haven't seen yet that they are a quantum leap forward in lifespan of parts. I like them, just keeping it all in perspective.


Del-ton barrels? I dont believe they are who most serious users would go to for technical information.

 

[azrocks]

Have you worn out some carbines? Your post indicates you aren't happy with the lifespan of a carbine. Someone I know of said hed "worn out almost 10 carbines", going about 20,000 rounds each before he decided to retire them and get a fresh one. I believe he said there were no parts replacements, and all were Colt guns. His use was for work.

What is the type of premature failure you are concerned with?

We don't know for certain if the functional reliability is the same, theres never been any side by side scientifically done tests that anyone knows of, or even any anecdotal reports that have used large enough numbers of both to make concrete conclusions from them. If someone could even say they used 20 guns of known quality and the parts lasted X number of rounds, it would be more than we have at the moment.

Pat Rogers wrote that he had two bolt lugs break off the BCM mid at 16400 rds that he had in his armory and was the subject of the Filthy 14 article, along with assorted other small parts like extractors and extractor springs at various intervals. He felt that was very acceptable life of all those parts. Then we have the owner of Battlefield LV that said his Colt M4 bolts were running to 50,000 rounds. So who can really say what the improvement is? If there is any really difference, it will pale to insignificance in the price of the handful of parts compared to the cost of the ammo to actually wear one out. If you like mids, buy one. Justifying it with lifespan of parts isnt really provable yet. I haven't seen yet that they are a quantum leap forward in lifespan of parts. I like them, just keeping it all in perspective.


Del-ton barrels? I dont believe they are who most serious users would go to for technical information.

First of all, there's more to consider than just wear. I thought I made that clear. Premature failure of parts due to manufacturing defects, unforeseen stress, or other factors is a potential issue.. an issue that may very well be exacerbated / accelerated by increased mechanical forces. But wear is a part of it, and as I said, it doesn't matter personally to me whether I can quantify it or not. If my rifle meets my expectations of acceptable reliability with a mid-length gas system, what would I possibly have to gain by going with a carbine (excepting AGB-equipped rifles)? Or, to view it the other way around: If I can get the same reliability as a carbine in a package that spares the gun some measure of abuse - regardless of how slight that measure may be - why would I choose otherwise?

No, I can't state with absolute certainty that my non-ABG-equipped rifles cycle more reliably than similar carbines. But in my experience, if I choose quality components assembled correctly and maintained properly (I'm sure guys following my malfunction thread will get a chuckle out of that one), they go bang every single time. As I don't expect to be shipped off to war with my personally owned-rifle any time soon, that experience is good enough for me (Though even if I was I'd still be happy with a mid-length). So what do I have in a middy? A rifle I'm perfectly content with regarding reliability, that absolutely does have at least some advantage in regards to wear and abuse. I don't care if that advantage is so minimal it makes zero statistical impact over the long run. It's still an advantage - without giving up anything - so why not? Plus there's the increased sight radius with FSBs.

I understand your reluctance to claim middy's are superior (rightfully). But you're getting so caught up in your argument that you're bordering on making the same claim for carbines.

 

[Malamute]

Actually, I was going to edit my last post and just say if anyone likes mids, that's plenty of reason to own one, and wasn't planning on reading this thread any longer (probably a relief to some).

I have no emotional attachment to any of them. Somebody asked a question about mids, and I made a point about what we do or don't know about the different ones, which many took as lack of knowledge or a put down of the idea somehow. Doesn't really matter to me what people buy and use, but many will convince themselves of things that just arent there, or make so little actual difference in real life as to be meaningless.

Cheers

 

[lobo9er]

Have you worn out some carbines? Your post indicates you aren't happy with the lifespan of a carbine. Someone I know of said hed "worn out almost 10 carbines", going about 20,000 rounds each before he decided to retire them and get a fresh one. I believe he said there were no parts replacements, and all were Colt guns. His use was for work.

What is the type of premature failure you are concerned with?

We don't know for certain if the functional reliability is the same, theres never been any side by side scientifically done tests that anyone knows of, or even any anecdotal reports that have used large enough numbers of both to make concrete conclusions from them. If someone could even say they used 20 guns of known quality and the parts lasted X number of rounds, it would be more than we have at the moment.

Pat Rogers wrote that he had two bolt lugs break off the BCM mid at 16400 rds that he had in his armory and was the subject of the Filthy 14 article, along with assorted other small parts like extractors and extractor springs at various intervals. He felt that was very acceptable life of all those parts. Then we have the owner of Battlefield LV that said his Colt M4 bolts were running to 50,000 rounds. So who can really say what the improvement is? If there is any really difference, it will pale to insignificance in the price of the handful of parts compared to the cost of the ammo to actually wear one out. If you like mids, buy one. Justifying it with lifespan of parts isnt really provable yet. I haven't seen yet that they are a quantum leap forward in lifespan of parts. I like them, just keeping it all in perspective.


Del-ton barrels? I dont believe they are who most serious users would go to for technical information.

first and foremost I have enjoyed this thread. its been very interesting both sides of each argument. And this purely friendly conversation.

I can't site articles or side by side tests. I believe they have been done but since I have no proof lets go with your side of it and say they may not exist or have never been done. The theory of less pressure, heat and force = less wear on moving parts has been thoroughly vetted on machinery. I am confident to say the same would be true for AR's. And there is a reason people claim they are "smoother". I will agree with you that carbine AR gas systems aren't falling apart nor would I likely wear one out in my life time. So less wear could be considered moot but to argue it could not exist and isn't provable goes against what I consider common sense about moving parts on just about all everything. when comes to steel on steel impact and movement... malamute come on bud not provable does it need a test to tell you wear would be less when the bolt is less violently blown back?

 

[RX-79G]

Totally aside from having a more sane gas system design, a middy looks more natural, has more room for accessories, more hand positions, a longer sight radius and the bayonet lug that isn't 100% decoration.

 

[RX-79G]

All the debate aside, if I had the kind of time and money to just build guns to play with, I think it would be pretty interesting to see how much you could tune down an AR using a bleed valve and lightweight springs/buffer. Ultimately, you only need enough recoil spring pressure to load the top round from a full mag, so if you can tune the gas down to just make the minimal spring pressure it would really change the way the rifle felt, and the gross weight.

But, when you use minimal gas to unlock the bolt, felt recoil might by slightly sharper as the breech will stay locked longer, transmitting more cartridge recoil instead of it getting lost in carrier movement. But it would be interesting.

The other thing it would be fun to play with is enlarging the bleed holes in the carrier, or adding another one further back that would open sooner. An experiment to try for someone with a crummy old carrier - especially if you can weld for trial and error.

 

[M1key]

Wait...didn't anyone mention bolt/carrier bounce? I knew I wasted 15 minutes reading this thread. LOL

Spent decades in the aerospace industry and semi-conductor business working with engineers. Lay them all end-to-end and they still couldn't reach a conclusion...

Agree with Ibmikey, I own them all and shoot them all and personally don't give a rip.

M

 

[lysanderxiii]

The problem with screwing around with port size is that you don't get something for nothing - a smaller port will still let the gas through, but now the gas has to move faster through the reduced port, which is going to be erosive and cause the gas to cool a bit just on the other side as it expands. I'm not saying it isn't a tool in the toolbox, but it has its own downsides. It's a bit like how pigtails don't do as much as they seem like they should - gas isn't so much traveling as pressurizing the system.

Increasing the mass is a good idea - unfortunately the move from rifle to carbine is usually a net reduction in mass.

The "perfect" 16" carbine is going to take advantage of all the off-the-shelf things you can do. Or, you can do none of them because they all increase the weight and you wanted a light rifle that is reliable enough.

Hmmmm..... No.

When gas passes through a restriction the speed will increase only to a point. That point is Mach 1. Making the restriction smaller does not increase the speed after that point.

The pressure differential across the restriction governs the size of the restriction at which Mach 1 is reached. The calculations are straight forward, and even with the low port pressures of 300 Blackout and the huge ports, they still are restricted to Mach 1.

That's why the pressure drops across the gas port.

Since the speed at which the gas is flowing is maximized, the only thing the influences the erosion of the port is the gas pressure and the temperature. Temperature in the bore remains fairly constant, about 1700 K, so the temperature drop remains fairly constant.

Moving the gas port forward only helps the pressure, as it gets lower the further from the chamber you get.

As to the temperature, you are alluding that there is more possibility of fouling due to to carbon precipitating out of the gas, but the temperatures in the mid-length system are still higher than the rifle-length system, in which there has been little problem with fouling.

 

[Malamute]

first and foremost I have enjoyed this thread. its been very interesting both sides of each argument. And this purely friendly conversation.

I can't site articles or side by side tests. I believe they have been done but since I have no proof lets go with your side of it and say they may not exist or have never been done. The theory of less pressure, heat and force = less wear on moving parts has been thoroughly vetted on machinery. I am confident to say the same would be true for AR's. And there is a reason people claim they are "smoother". I will agree with you that carbine AR gas systems aren't falling apart nor would I likely wear one out in my life time. So less wear could be considered moot but to argue it could not exist and isn't provable goes against what I consider common sense about moving parts on just about all everything. when comes to steel on steel impact and movement... malamute come on bud not provable does it need a test to tell you wear would be less when the bolt is less violently blown back?

Completely agree with your perspective, good post.

Yes, I think the information indicates an improvement, and that's why I prefer mids in the less than 20" guns.

I mainly came back to this thread to apologize for being more adamant in expressing my point than was necessary, and hope the discussion proceeds with more interesting posts. Theres been several I enjoyed and learned from (despite what it may have seemed).

The post I quoted above is the only one Ive read in this thread since yesterday, or whenever I last posted. I'll catch up at some point.

 

[lysanderxiii]

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. Anyone with $150 for Quickload can draw up the appropriate pressure curve, and anyone who finished high school physics can determine the resulting forces kicking around the action. Given an existing system, this is all a person needs to run the numbers on the action.

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 (college physics and calc 1 & 2). This kind of person could design the system from scratch - which is to say the folks who came up with it 60yrs+ ago aren't so unique in their ability to run numbers.

What I can say, after working with some firearms design firms in my own professional engineering career, is the 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. There IS a downside, of course, meaning lower quality engineers can accomplish far more than higher quality engineers of yester-year, but at the same time, it also means even the slack-jawed intern fresh out of college can run computations and calculations which were effectively prohibitive even for the top minds a few decades ago.

The good news - however, is that fixing the issue is incredibly simple. Want the performance of a mid-length but have a carbine length? Throw on an adjustable gas block and you can throttle the pressure down in a carbine system to the same level as a mid. Want the same bolt speed and cyclic rate as a rifle length system out of your carbine? Throw in an H3 buffer.

But the real answer is this: There are multiple combinations which operate with high reliability and extreme longevity. Not all AR's have to have the same bolt speed and cyclic rate as a 20" M16A1. Ford F-150's make it down the road even though they might offer 3 different motors in the same body class, and all have their own specific attributes. AR's will reliably operate with multiple different gas system combinations.

Modeling the gas system of an M16 is easily in the grasp of a high school physics graduate?

Really?

Then this, this and this should end the argument.

Now, everybody, go calculate the bolt velocity with the equations and the information contained to the above reports for a standard carbine and mid-length gas system with a 16" barrel. You can find pressure-time curves for typical 5.56/.223 ammunition on the web for various barrel lengths. The scale of these graphs is a little coarse, but it is good enough.

 

[RX-79G]

When gas passes through a restriction the speed will increase only to a point. That point is Mach 1. Making the restriction smaller does not increase the speed after that point.

That gas is already moving faster than Mach 1. It pushed a bullet to Mach 3.

 

[lysanderxiii]

That gas is already moving faster than Mach 1. It pushed a bullet to Mach 3.

No, just no.

One -The speed of sound is temperature dependent, the gas temperature is 2700 degrees K. At that temperature the speed of sound is 5500 fps. How fast is your bullet going? It ain't going 16,500 fps, that's for sure.

(Calculate the speed of sound)

Two - That gas is not restricted.

Three - That gas is traveling in the direction of the bullet, not up the gas port, 90 degrees from its original direction of flow (or 135 degrees in an AK).

Four - Go study compressible gas flow.

[EDIT: Once the gas exits the muzzle, it pushes the ambient air out of the way, the ambient air is 25 +/- 20 degrees, that air gets to move at super-sonic velocities....]

 

[lysanderxiii]

So when you used them, did it help you understand why military rifles with adjustable gas systems don't work by restricting gas? They work by dumping the extra gas.

Again, no...

Rifles with restricted gas flow prior to the gas cylinder:

FG-42
BREN
VZ-26 & 30
MAG-58
M-249
SVT-40
HS-404
and that's off the top of my head.

Rifles that dump gas at the gas cylinder or in the cylinder before completion of work:
FN FAL

 

[RX-79G]

One -The speed of sound is temperature dependent, the gas temperature is 2700 degrees K. At that temperature the speed of sound is 5500 fps. How fast is your bullet going? It ain't going 16,500 fps, that's for sure.

That's all fine, but you're basically saying that the gas is moving at 5500fps when it hits the restriction, and therefore can't go any faster.

So is it traveling at 5500fps when it hits the gas adjustment, and where did you find that number?

 

[lobo9er]

Completely agree with your perspective, good post.

Yes, I think the information indicates an improvement, and that's why I prefer mids in the less than 20" guns.

I mainly came back to this thread to apologize for being more adamant in expressing my point than was necessary, and hope the discussion proceeds with more interesting posts. Theres been several I enjoyed and learned from (despite what it may have seemed).

The post I quoted above is the only one Ive read in this thread since yesterday, or whenever I last posted. I'll catch up at some point.

nothing better than debating and talking guns.

 

[Maj Dad]

Let me throw a little gas on the fire with my 18" 1/7, ss bbl with Wylde chamber & a rifle length gas system. Did some reading, and my take on it all is there ain't a lot of difference when the rubber hits the road... Thoughts?

 

[lysanderxiii]

Restrictions in pressure systems create "pressure drop."

OK, if thats not a correct assumption on my part about the pressure not being reduced, if thats an unqualified improvement, why wasn't it used to reduce gas pressure in the M4s? I'm asking if its a definite improvement in overall function and reliability. If making the port size of an M4, or using an adjuster to 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. For a civilian use gun, its probably not as much of an issue as it is with a military gun that it work reliably without fiddling with the gas adjustment. Curious, not being confrontational. Educate me.

Gas volume and pressure.

The gas in the bolt carrier (piston/cylinder) has to be a a certain pressure to accelerate the carrier/bolt/buffer combination to achieve the necessary momentum to work the system. As gas expands, the pressure drops, unless more gas is added, therefore you need a gas flow to maintain the pressure.

Reducing the port diameter will reduce the pressure in the gas tube, but it will also reduce the gas flow.

There is only so small you can make a gas port before the mass flow to the cylinder is too small and there is insufficient energy to work the system. (AKA "undergassed")

And to the bold - they did do that within the confines of the system. The smaller gas port allows the pressure in the gas tube to stay very close to the original M16 design, the shorter gas tube means there is less volume for that volume to fill, but there is still a small deficit in mass delivered.

This is why the buffer on a M4 is lighter than an M16. Because the total gas mass delivered to the cylinder is lower, the total energy in the cylinder is less, so in order to get the velocity back up to where it works reliably the mass of the carrier and buffer has to drop

 

[RX-79G]

Let me throw a little gas on the fire with my 18" 1/7, ss bbl with Wylde chamber & a rifle length gas system. Did some reading, and my take on it all is there ain't a lot of difference when the rubber hits the road... Thoughts?

Your rifle sounds like it has about the lowest working gas pressure possible. Is the port 20" size, or even larger?

 

[lysanderxiii]

That's all fine, but you're basically saying that the gas is moving at 5500fps when it hits the restriction, and therefore can't go any faster.

So is it traveling at 5500fps when it hits the gas adjustment, and where did you find that number?

Lets look at one gas molecule.

The gas molecule is traveling in the direction of the bullet (axially).

What is the velocity of the gas molecule in the direction of the gas port is pointed in (radially, ie 90 degrees to the barrel bore)? Zero, otherwise it would be speeding through the walls of the barrel.

The gas molecule encounters a hole in the wall of the barrel (the gas port) with a low pressure on the other side, so it accelerates from zero (radial) velocity to Mach 1 at the restriction. Yes, it looses its axial velocity by bouncing off the front wall of the gas port, that's why gas ports erode the way they do.

Here is a velocity map of an M4 gas port and gas tube:


You can see the velocity in the barrel is around 900 m/s and it accelerates to around 1500 m/s in the gas port. You can also see the restriction point at the bottom of the gas port. The gas ahead of the gas port are the gas molecules that missed the exit and are still speeding down the barrel chasing the bullet.

Where did I find the speed of sound? From the link provided.

 

[UltraT]

Does the gas port on a carbine length setup erode significantly faster than the port on a rifle length or mid length setup?

 

[RX-79G]

Lets look at one gas molecule.

The gas molecule is traveling in the direction of the bullet (axially).

What is the velocity of the gas molecule in the direction of the gas port is pointed in (radially, ie 90 degrees to the barrel bore)? Zero, otherwise it would be speeding through the walls of the barrel.

The gas molecule encounters a hole in the wall of the barrel (the gas port) with a low pressure on the other side, so it accelerates from zero (radial) velocity to Mach 1 at the restriction. Yes, it looses its axial velocity by bouncing off the front wall of the gas port, that's why gas ports erode the way they do.

Here is a velocity map of an M4 gas port and gas tube:


You can see the velocity in the barrel is around 700 m/s and it accelerates to around 1500 m/s in the gas port. You can also see the restriction point at the bottom of the gas port.

Where did I find the speed of sound? From the link provided.

1500 m/s is 4921 fps, which is below 5500fps. So you're saying that the speed of sound at temp is the issue, or it isn't? Is 4921 close enough to 5500 to mean that restricting the port below .06" than expanding back to .120 in the gas tube has no consequences for particulate condensing out of gas?

The primary thing I have been trying to get at is that the AR15 bolt and gas system was designed around a certain amount of flow and pressure that is found in the 20" version. Tapping the gas other places, using a smaller port, then reducing that diameter further is not going to duplicate the design parameters. And there has to be a point when a gas restriction is simply too small for good function and longevity.

 

[lysanderxiii]

Does the gas port on a carbine length setup erode significantly faster than the port on a rifle length or mid length setup?

Yes, it erodes faster.

Whether or not it is significant, I can't say.

 

[UltraT]

Just on intuition, I would think it would wear at the same rate as a 14.5" carbine using the same gas port diameter. The extra barrel length at the end wouldn't affect the characteristics of the gas as it's hitting the gas port, or am I mistaken?