Carbine Length Gas Systems?

[TMiller556]

I know this is a basic question but I always hear that a carbine length gas system timing is off and less reliable than a longer gas system and I never understood this. Can someone please explain to me if this is true and why. Thanks

 

[RP88]

it has a lot to do with the gas system's length versus barrel length. Short gas lengths are for short barrels.

The carbine system was designed for a barrel even shorter than the current 14.5" barrels i.e. 10.5-12" barrels. These barrels allow enough gas to escape from the muzzle, and leaving enough to smoothly cycle the action.

In the 14.5" barrels, more gas is trapped in the barrel by time it begins to cycle. This leads to a very abrupt, harder 'push' into the gas key (since this leads to the extra gas being sucked into the gas tube to be used to work the action), and therefore compresses the buffer spring quicker and more abruptly, as the bolt carrier group flies back with way more gas giving it momentum. And as physics dictates, every action elicits an equal opposing action; the bolt will slam forward faster and this leaves the gun at risk of misfeeding, breaking bolts, added wear, etc. etc.. This is why they generally use H buffers in carbines: to make up for the added push from the extra gasses that go back into the gun instead of escaping out the muzzle. This problem is further exacerbated in the 16" barrels that use carbines.

This is also why midlength and spectre systems are being touted as superior in carbine vets due to the extra length in the gas system being more natural to the 14.5-16" barrels they usually use.

 

[HOLY DIVER]

yea...ummm what he said...that covers it,very good reply RP88

 

[TMiller556]

it has a lot to do with the gas system's length versus barrel length. Short gas lengths are for short barrels.

The carbine system was designed for a barrel even shorter than the current 14.5" barrels i.e. 10.5-12" barrels. These barrels allow enough gas to escape from the muzzle, and leaving enough to smoothly cycle the action.

In the 14.5" barrels, more gas is trapped in the barrel by time it begins to cycle. This leads to a very abrupt, harder 'push' into the gas key (since this leads to the extra gas being sucked into the gas tube to be used to work the action), and therefore compresses the buffer spring quicker and more abruptly, as the bolt carrier group flies back with way more gas giving it momentum. And as physics dictates, every action elicits an equal opposing action; the bolt will slam forward faster and this leaves the gun at risk of misfeeding, breaking bolts, added wear, etc. etc.. This is why they generally use H buffers in carbines: to make up for the added push from the extra gasses that go back into the gun instead of escaping out the muzzle. This problem is further exacerbated in the 16" barrels that use carbines.

This is also why midlength and spectre systems are being touted as superior in carbine vets due to the extra length in the gas system being more natural to the 14.5-16" barrels they usually use.

oh ok, I get it now. So if you use say a 10 inch barrel with a carbine length gas system, would the parts be more likely to wear than a carbine length gas system with a 16 inch barrel?

 

[RP88]

no, since there will be less gas going back into the carrier group to basically bang the hell out of it. Instead, a lot of the gas will be able to expand and exit the muzzlle. This, however, leads to less ballistics; if the propellant expands and escapes the gun sooner, then less of it was used to drive the bullet out. That is why people 'whine' about ballistics from 14.5" barrels.

This means that the action will cycle more smoothly and naturally compared to what happens when you basically over-gas the system, which is what happens (relatively speaking) to short gas systems with too long of barrels on them; too much gas is forced into the receiver, this makes the bolt fly the heck back more than it would normally in, say, a midlength or rifle-length with a 20" barrel.

And yes, doing the exact opposite can have the opposite effect; undergassing the system with too long a gas system for your barrel and having too heavy a buffer can lead to the gun being mis-timed in the other direction, and lead to the same problems, but this time in reverse (not cycling hard enough to clear the receiver, catch new rounds from the mag, etc.)

 

[john5036]

In regards to reliability, these issues are NOT urgent enough that you should feel dissuaded from going the carbine route. We are talking about a LOT of use for these problems of wear to manifest in a carbine system.

The benefits of a mid-length and longer are more practical in nature (longer sight radius, extra room on handguard for support shooting, softer recoil), not so much in reliability. A quality AR is a quality AR.

 

[MachIVshooter]

RP88 nailed it.

That said, the problem is more theoretical than real; You're not likley to see any damage to your 16" carbine so long as you do normal maintenance, unless you're one of those people who can afford to fire tens of thousands of rounds every year.

I personally prefer the Mid length system, but it has nothing to do with the slightly smoother/gentler cycling; I just think a 16" barrel with a carbine handguard looks silly.

 

[Tirod]

Gas length is the timing for the AR. You don't see carbine gas on 20" rifle barrels.

What happened is that after the Army approved the M4, the public wanted a look alike, and got it - with legal 16" barrels. It does ok, but the makers saw the writing on the Customer Service wall, and began offering mid length, the more appropriate gas length for the 16".

The important issue is that the effective gas length for any AR is measured from the muzzle back - about 5-7 inches. Take a longer look, that's the critical dimension to make any of them work right. That gives the action just enough gas to cycle, and cuts it off by the bullet leaving the muzzle. It's the real gas length of the system, and they are all about the same - regardless of the amount of barrel length.

The idea port size can compensate doesn't work, it just allows cheap plinker ammo to cycle. Big ports in AR shoot a lot harder with milspec ammo, and it goes right back to being too much gas to do the job. Then other issues crop up, like poor extraction, or bolt over the ammo, because cycling speeds get too high. Then a heavier buffer is used, which causes poor cycling with cheap plinker ammo.

You can balance the tradeoffs, the real issue is that the AR is built to either handle one or the other. Shotguns use some creative dynamic gas port mechanisms to shoot light and heavy loads, but the AR doesn't. Build it one way and just shoot that, you get reliable action. Shoot stuff it's not tuned for, you get problems, and cheap import fodder is creating a problem where none should exist. It's not dependable.

 

[sansone]

excellent thread, where's the popcorn

 

[browneu]

The light bulb finally went off in my head after reading countless threads of dwell time, gas systems, and cycling.

I thought it was from the chamber to the gas port and now finally realize its from the gas port to the muzzle.

Sent from my LG-P999 using Tapatalk

 

[RP88]

In regards to reliability, these issues are NOT urgent enough that you should feel dissuaded from going the carbine route. We are talking about a LOT of use for these problems of wear to manifest in a carbine system.

The benefits of a mid-length and longer are more practical in nature (longer sight radius, extra room on handguard for support shooting, softer recoil), not so much in reliability. A quality AR is a quality AR.

this, too.

The only thing that really matters in keeping a carbine reliable (at least as far as the action and cycling go) is choosing the right buffer. A lot of people underrate how important that $10-20 piece is to the system, but in all actuality buffers are what can make, break, fix or regulate a good or bad barrel/gas length combo.

 

[Mako_44]

RP88,
The M4 gas system is not the same as the old XM177 or CAR-15/16 gas systems. It is purpose built, when Colt was working on the project they subcontracted the gas system and the buffer spring and buffer assembly analysis used in development. The environmental testing chambers were tied up (actually being rebuilt) at the time for the full range operating conditions and the first development was done with designed loads from Olin to simulate temperature, pressure, humidity and altitude conditions.

It isn't the amount of the gas that is the issue, it's the pressure. The pressure vessel volume and pressure do have a direct relationship but the time element in the gas cycle is actually the driving factor. The current ball powders spike and the further up the barrel you port the gas the flatter the pressure ramp is and the impulse is definitely softer.

The original 20" design took the gas 4 inches further up the bore, the original shorties took it 4 inches further back, shorter versions took it even closer. The Mid length gas systems take it 2 inches further out than the M4 gas system.

The M4 was basically attenuated not "balanced" with the buffer, it works but it isn't ideal.

For instance the current M855 ball has an operating pressure of 55kpsi as compared to the old M193 ball at 52kpsi. Barrel length is "irrelevant" in those pressures because the peak is about 8 inches down the barrel. The further the gas travels beyond this point the flatter the rate curve and it is actually dropping off in pressure. The Gas is still expanding and the bullet can still accelerate because there is still pressure behind it.

The faster twist rates on the barrel and the heavier bullet means the current M4 system deals with higher pressure than the old systems ever saw.

In your last post you actually said a mouthful...it really is the buffer that allows the weapon to work. And, they can be purpose chosen based on the loads, barrel twist rate and operating theater.

Good thread,
Mako

P.S. there are actually differences in the Bolt and Bolt Carrier Assembly as well. You would have to have a vintage system to see the differences. The commercial shorties and things you see now are actually the new style. If you have access to and old Colt SP1 carbine you can see the difference but I wouldn't trust any other manufactured one to be 100% sure it was really the old system. If you have one check out the gas port, the gas tube I.D., the buffer assembly and the Bolt/Bolt carrier.

 

[MachIVshooter]

I thought it was from the chamber to the gas port and now finally realize its from the gas port to the muzzle.

It's both, and is not a set number. It's going to depend on cartridge pressure and gun design, and to some extent, bullet weight. M1 Garands have the port nearly to the end of the muzzle, while on a Desert Eagle it is directly in front of the chamber.

On most guns, however, you will find the port about 2/3 of the way down the barrel.

A few guns have an adjustable gas system that requires the settings be changed for different types of ammunition, or to adjust for use with a supressor (dwell time increases significantly)

Basically, what matters is:

1)That the bullet is far enough down to barrel that chamber pressures are safe and the bolt won't open until the round exits the muzzle
2) pressure when the bullet passes the port is sufficient to work the action
3) pressure is not excessive

These things are determined by port location versus barrel length, and by the size of the port. Most fixed gas systems are over ported a little so that they'll function with low powered ammo, at the cost of somewhat violent cycling with more powerful loads. This is akin to using a lighter recoil spring in a recoil-operated gun to ensure cycling with a wider range of loads.

The problem with the M-4gery is that the gas system was designed with the dwell time of a 14.5" barrel, not a 16 incher, while the mid-length system is specifically suited to the 16" barrel.

 

[browneu]

MachIVshooter

Thanks for the clarification and the light bulb just exploded. Perhaps I'm better just shooting the gun.

I like my midlength rifles and this thread helped give me some understanding why, meaning I'm not crazy about the softer recoil.

Sent from my LG-P999 using Tapatalk

 

[RP88]

It isn't the amount of the gas that is the issue, it's the pressure

That was what I was trying to imply. I guess I should have used the word in there. I do agree.

Also, I thought that the carbine system used in the M4 was actually adopted (or, at least adapted) from the CAR-15/XM177 derivatives, and possibly guns like the Mk-18 and so forth that also used very similar parts. This was what I heard, though, so what you said is very interesting.

I always thought that a lot of the M4 was similar to the rest of the CAR-type guns due to parts commonality being a huge drive in how they designed the M4.

 

[MachIVshooter]

I thought that the carbine system used in the M4 was actually adopted (or, at least adapted) from the CAR-15/XM177 derivatives

That's my understanding of it. Same length, just recalibrated for the slightly longer barrel and dwell time. I think it included a smaller I.D. gas tube.

 

[Bartholomew Roberts]

The M4 gas system is not the same as the old XM177 or CAR-15/16 gas systems. It is purpose built, when Colt was working on the project they subcontracted the gas system and the buffer spring and buffer assembly analysis used in development

Unless you consider the buffer and action spring a part of the gas system, the only difference between the XM177 and an M4 is the dwell time after the gas port and the size of the gas port.