AR Gas Enigma

[Coal Dragger]

I want durability in a fighting implement. Over gassed AR’s break bolt lugs, and extractors.

I never used weak .223 in a combat zone, and I’m not going to buy any or reload any now.

 

[MachIVshooter]

My Colt 6920 wasn't over gassed.

There are suppressed ARs with non adjustable gas blocks that aren't over gassed.

It's all about the gas port diameter.

In the context of the nitpicking of a couple thou port diameter Coal Dragger is doing they are. In the real world, 16" guns with carbine gas using 14.5" port diameters just have a little higher bolt speed than rifles with proper dwell and port ratios. As for suppressed guns, you might not notice it without a high speed camera, but trust me, weapons with a fixed aperture block and appropriate port to run unsuppressed have significantly increased bolt speed with a can. You may not realize it with a semi-auto, but pop that thing on an M16 lower in go fast mode, and the failures ensue. I have done so with no less than a dozen uppers that run fine with suppressors single fire, and run fine full auto unsuppressed, but can barely get 2 or 3 rounds out without a jam running suppressed in FA.

At the end of the day you either want an AR that is made to run 5.56mm NATO optimally or one that is made to run .223 Remington optimally.

At the end of the day, you want a rifle that will run, period. The actual chamber pressure difference between 5.56 and .223 is due to leade, and it's pretty small, especially with .223 in a 5.56mm chamber. If your rifle cycles with M193 or M855 but won't run with various .223 loads, it's barely functioning. I would never just live with a rifle that only ate brass cased ball ammo. Nearly every AR I've bought or built would run every type of ammo fed to it, and the few that wouldn't got ports opened up. The only ammo currently on the market that I'll dismiss as underpowered is Tula .223, which still cycles most of mine, but won't lock the bolt open on my 7.5" or 10.5" unsuppressed.

It's OK to have .22s that won't run certain brands or bullet profiles. Not acceptable with an AR.

Over gassed AR’s break bolt lugs, and extractors.

You do understand that the gas system has zero bearing on bolt thrust, yes?

 

[Coal Dragger]

You do understand that unlocking speed from higher carrier velocity has a negative impact on bolt life and extractor life? The US military has done a lot of studies on this and has replacement schedules more or less according to barrel length. The shorter the barrel, and larger the gas port the shorter the bolt life.

 

[giggitygiggity]

You do understand that unlocking speed from higher carrier velocity has a negative impact on bolt life and extractor life? The US military has done a lot of studies on this and has replacement schedules more or less according to barrel .

Coal Dragger, do you have links or can you provide the U.S. military’s studies? I’d be interested in seeing them.

I’ve never heard of maintenance schedules corresponding to barrel length. Do you know which branch(es) does that? In the Marine Corps, the maintenance plan is essentially to fix what is broken when it breaks, conduct limited technical inspections and pre-firing inspections prior to ranges and fix/replace any broken/worn parts or other areas of concern, and to clean weapons spotless after ranges/field exercises and during monthly cleanings. Some weapons look like they are “in the white” from having been used and others are pristine. They get replaced when they break beyond repair, not when they reach a certain age. Not saying you’re full of BS, but I have not heard of barrel length having anything to do with maintenance schedules.

 

[someguy2800]

I'm very skeptical the military could possibly keep track of round counts anyway.

 

[Coal Dragger]

Coal Dragger, do you have links or can you provide the U.S. military’s studies? I’d be interested in seeing them.

I’ve never heard of maintenance schedules corresponding to barrel length. Do you know which branch(es) does that? In the Marine Corps, the maintenance plan is essentially to fix what is broken when it breaks, conduct limited technical inspections and pre-firing inspections prior to ranges and fix/replace any broken/worn parts or other areas of concern, and to clean weapons spotless after ranges/field exercises and during monthly cleanings. Some weapons look like they are “in the white” from having been used and others are pristine. They get replaced when they break beyond repair, not when they reach a certain age. Not saying you’re full of BS, but I have not heard of barrel length having anything to do with maintenance schedules.

I’ll see if I can find it, the topic came up in a lecture Pat Rogers (RIP) was giving, and he cited it. If I had to guess the data was probably compiled by Crane Naval Surface Warfare Center.



At about 21:00 Pat goes over parts that break.

Whole video is worth your time.

 

[MistWolf]

In the context of the nitpicking of a couple thou port diameter Coal Dragger is doing they are. In the real world, 16" guns with carbine gas using 14.5" port diameters just have a little higher bolt speed than rifles with proper dwell and port ratios.

Nitpicking or not, my 6920 was not over gassed. I've worked with over gassed ARs and have spent time experimenting with AR tuning using an adjustable gas block. After everything I've read online about 16" barrels with carbine gas systems having harsh recoil, I was surprised at how smooth the 6920 felt.

As for suppressed guns, you might not notice it without a high speed camera, but trust me, weapons with a fixed aperture block and appropriate port to run unsuppressed have significantly increased bolt speed with a can.

Yes. Yes, they do. But in your earlier post, you said any fixed port suppressed.

With the right gas port, an AR will not be over (or under) gassed.

 

[MachIVshooter]

You do understand that unlocking speed from higher carrier velocity has a negative impact on bolt life and extractor life? The US military has done a lot of studies on this and has replacement schedules more or less according to barrel length. The shorter the barrel, and larger the gas port the shorter the bolt life.

It's not the lugs which suffer from high unlocking speed; it's the bolt body at the cam pin. Lugs break from bolt thrust.

As for the extractor, the earlier unlocking on carbines with higher chamber pressure is what strains them. Higher chamber pressure means the case is more firmly adhered to the chamber wall. The case requires X amount of force to be yanked from the chamber, regardless of the carrier velocity. Port pressure, gas volume and extraction speed matter not. This, of course, completely ignores that it's the extractor snapping over the case rim which imparts the majority of wear.

Yes. Yes, they do. But in your earlier post, you said any fixed port suppressed.

With the right gas port, an AR will not be over (or under) gassed.

How many production barrels, uppers or complete rifles are ported to run suppressed, leaving them undergassed without a can? Even rifles billed as being suppressor ready are equipped with adjustable gas blocks for that purpose, not undersize ports. I don't see that it's necessary to qualify that statement.

IME, most ARs will run fine in semi-auto with a gas port sized for unsuppressed port pressure & dwell time. I've only encountered problems in full auto. .308 ARs are another matter, many of them unable to run reliably with a can unless you install an adjustable gas block.

 

[Coal Dragger]

Higher chamber pressure during unlocking also damages the bolt lugs, remember those high carrier speeds that beat up the bolt at the cam pin mean the bolt is trying to unlock prematurely against a fair amount of bolt thrust. It’s the same timing issue that is killing the extractor that is prematurely cracking bolt lugs. You’re already recognizing that it murders extractors first, but it will also murder the bolt lugs. Imagine the force being applied to the lugs as the bolt is attempting to rotate while you still have too much bolt thrust from higher chamber pressure than designed for. Why is this happening? Because the timing is way off, too much gas, at too high a pressure, too soon in the cycle of operation.

Peak bolt thrust shouldn’t be any higher or lower given identical chambers and ammo between test subjects. If you cut off all gas and ran the rifle as a single shot, the bolt in a 10.5” barrel should last just as long one in a 20” barrel.

If you then put the gas to them the 10.5” is going to eat its bolt much much sooner given the same firing schedule. It will be a race to see what fails first: the lugs cracking or the bolt body breaking in half at the cam pin. I’d bet $10 that careful inspection with a microscope, or radiograph will show crack propagation in the lugs next to the extractor before the cam pin hole starts to fail. Maybe not enough to shear a lug, but the damage will be there.

 

[someguy2800]

I have a hard time believing that overgassing causes bolt lugs to crack

 

[Coal Dragger]

That’s because you have a very poor understanding of how gas operated firearms work.

 

[someguy2800]

Well one thing is for certain, one of us is talking out their butt.

 

[Coal Dragger]

Peruse this thread and look for the post by Eric D. a mechanical engineer with his company signature at the bottom. He explains it more eloquently. Basically the bolt has to deal with both longitudinal force (bolt thrust), and lateral shear forces as the bolt lugs move during unlocking along with a few other stresses he covers briefly.

http://www.m4carbine.net/showthread.php?124056-Bolt-Failure-Points

This is also a useful thread, particularly page 2 post by markm. Hopefully the pictures work for you since they detail peak stress locations on an AR bolt. Notice the highest load areas, and most prone to crack propagation, even in a normal M16, are the portions of the bolt lugs that end up as the bearing surface during locking and unlocking. Almost always the two lugs adjacent to the extractor because they end up seeing more thrust loading, and are weaker in the lateral direction as well because the bolt is cut out to accommodate the extractor.

http://www.m4carbine.net/showthread.php?63182-locking-lug-integrity-differences-between-m4-m16/page2

Granted we’re getting into the weeds here, and it takes a pretty extreme example of an AR to see drastic catastrophic bolt failures super prematurely. Something like a MK18 CQB carbine running a 10.5” barrel is going to brutalize bolts; short gas system, short barrel without a lot of dwell time so the gas port has to be hogged out. You get the gas at higher pressure and temperature because the gas system is short, and a higher volume because there’s not enough dwell time to allow a smaller port to cycle the gun. So you will see that carrier starting to unlock faster in the cycle of operation while thrust forces are still high. You get cracks in the lugs and cracks at the cam pin hole.

That same bolt in a 20” rifle with a rifle length gas system will run for a long long time. Probably the life of the barrel. I will admit that even an over gassed 20” rifle will never display the problems an over gassed shorty will. That longer gas system is far superior for keeping the gun timed correctly as Stoner originally designed it.

 

[zaitcev]

Higher chamber pressure during unlocking also damages the bolt lugs, remember those high carrier speeds that beat up the bolt at the cam pin mean the bolt is trying to unlock prematurely against a fair amount of bolt thrust. It’s the same timing issue that is killing the extractor that is prematurely cracking bolt lugs. You’re already recognizing that it murders extractors first, but it will also murder the bolt lugs. Imagine the force being applied to the lugs as the bolt is attempting to rotate while you still have too much bolt thrust from higher chamber pressure than designed for.

Th vivid picture above appears to assume a model that does not correspond to how an AR works in reality. The chamber pressure is already ambient by the time bolt ever starts to rotate. You'd need a barrel 10 feet long for it not to be, considering how fast a bullet leaves the barrel, the speed that the carrier attains, and the length of the straight area of the cam slot. The only thing that beats up an overgassed AR is the pressure in the carrier plenum. There is never such a thing as "higher chamber pressure during unlocking".

 

[Coal Dragger]

Expanding gas from burning gun powder moves at roughly 5X the speed of the bullet out of the muzzle after the bullet clears the muzzle.

Remember those time, speed, distance calculations in school? Usually word problems.

Do one for expanding powder gas needing to cover the total distance of the gas system of a carbine from the gas port back, versus the time needed for the bullet to exit the barrel from the gas port forward. Now tell me there’s no chamber pressure at all when gas hits the bolt carrier and it initiates movement.

A useful step by step on what is going on:

https://www.ar15.com/forums/AR-15/H...d-how-everything-works-in-harmony-/66-266108/