Honest Question: Mid-Length vs Carbine Length Gas Systems?

[Pappabear]

99% of the AR owners will never wear their guns out. I never worry about shooting my barrels hot and letting it rip. Buy what you want and shoot it as often as possible.

PB

[mack7.62]

I find it very hard to believe that a change in gas system would consistently increase barrel life. Throat erosion has absolutely no correlation with what occurs when the gas reaches the bolt carrier.

An increase in bolt life is completely reasonable. but it's not like bolt breakage is common.

I always understood that M4 barrels wear out at the gas port not the chamber, port erosion is going to be faster the higher the pressure and as port opens up parts wear increases. The port pressure of the M4 at 7.8” from the bolt face is 17,000 psi, while port pressure at 13” from bolt face of the M16 is 10,000 psi, I don't know what it is for mid but it's got to be somewhere in the middle so all other factors being equal a mid barrel will last longer. And bolt breakage is common in high round counts.

[vicious_cb]

I always understood that M4 barrels wear out at the gas port not the chamber, port erosion is going to be faster the higher the pressure and as port opens up parts wear increases. The port pressure of the M4 at 7.8” from the bolt face is 17,000 psi, while port pressure at 13” from bolt face of the M16 is 10,000 psi, I don't know what it is for mid but it's got to be somewhere in the middle so all other factors being equal a mid barrel will last longer. And bolt breakage is common in high round counts.

I dont know about you but I haven't heard of anyone junking barrels because their gas ports eroded, they are junked because they start keyholing (rare) or accuracy drops below a certain threshold. Even .mil barrels are getting junked because of throat erosion using go/no-go gauges.

Honestly junking a barrel because of an eroded gas port sounds like a waste, if its still shooting good throw a heavier buffer in and keep shooting till accuracy drops off.

[grizzman]

Yes, port erosion at the carbine location does occur at a faster rate than it does at mid and rifle locations. Is a barrel considered worn out due to a measurement of the gas port (never heard of it), by measurement at the throat and muzzle (yep), or by measurement of group sizes (yep)?

Papabear's statement is absolutely correct. Barrel life outside of a competition rifle is basically meaningless, and a service rifle barrel will be replaced loooong before bolt life comes into play.

[vicious_cb]

Yes, port erosion at the carbine location does occur at a faster rate than it does at mid and rifle locations. Is a barrel considered worn out due to a measurement of the gas port (never heard of it), by measurement at the throat and muzzle (yep), or by measurement of group sizes (yep)?

Papabear's statement is absolutely correct. Barrel life outside of a competition rifle is basically meaningless, and a service rifle barrel will be replaced loooong before bolt life comes into play.

Probably because you can't, if you look at how gas port erosion happens its oblong and looks like a streak that heads off towards the muzzle.

I dont know if its because bolt manufacturing just got better over time even using the same materials but Ive never broken a bolt that wasnt already defective to begin with(you'll know its a bad bolt within a few thousand rounds). All my retired barrels retired with their original bolts that lasted the life of the barrel.

[sinister]

... I have seen no significant difference between properly gased car gas length systems and mid lengths. Maybe a some of my smaller gas port midlengths shoot a tiny bit smoother but you probably wouldnt notice the difference unless you were doing a back to back comparison.

Basically I shot out all my 16" car barrels from S&W to Colt and one 14.5" LMT car M4 style barrel that I built in the early to mid 2000's and replaced them all with midlengths, and I have noticed basically nothing in terms of real world shootability, durability or parts life.

Same here -- maybe a half-dozen barrels.

On high round count GI M4s and M16s I broke a single bolt, an extractor pin, and had severe flame-cutting through the muzzle end of the slots in a single A2 muzzle compensator over tens of thousands of rounds.

[ST911]

Port erosion is irrelevant to most users, and barrels go when they don't shoot to requirements anymore. That said, it is one of several indicators useful in assessing barrels doing precision or test-article work. If you have a bore scope, it can be interesting to watch throat and port erosion over time.

[LivingtheHistoryM1]

So, to understand the summary, I would probably not see much of a difference, but to buy what I can get my hands on and run with it, knowing that mid-length will likely last a bit longer. Understandingly, that's going to be very dependent on barrel quality. I have already purchased a BCM bolt, so I doubt that'll be a failure point.

[markm]

Port erosion is irrelevant to most users, and barrels go when they don't shoot to requirements anymore. That said, it is one of several indicators useful in assessing barrels doing precision or test-article work. If you have a bore scope, it can be interesting to watch throat and port erosion over time.

Does the port actually open up in diameter though? I know there's the tear drop erosion in the actual bore, but I've always assumed that the port diameter stayed the same or damned near the same at the top of the barrel for ever.

[Todd.K]

Port erosion is irrelevant to most users, and barrels go when they don't shoot to requirements anymore.

Port erosion can be a part of accuracy degradation. Generally as excessive copper fouling.

But there is no significant difference for the individual buying his own ammo to shoot out his own barrel.