Difference between gas system lengths?

Decent enough explanation: https://www.shootingillustrated.com/articles/2014/9/24/ar-gas-system-lengths-explained/

For the military M4 carbine the barrel length is 14.5" and for that barrel a carbine length gas system is required since anything longer would interfere with mounting a grenade launcher. Civilian AR carbines typically have barrels 16" or longer because anything shorter requires a short barrel rifle tax stamp.

For a 16+" barrel length, a mid-length gas system has some advantages. It moves the front sight/gas block farther forward providing for a longer sight radius, and it allows for a longer hand-guard providing more flexibility in mounting accessories. The recoil, although not very great with the 223 Rem/5.56x45 round, is also reduced a bit with a longer gas system length.

You can read the extensive amount of data that exists of why the different lengths exist. There are several factors you can use to customize your firearm to the experience you're looking for. However, from a practicality standpoint, reputable barrel makers have developed the right combinations of barrel length, gas port diameter and gas system length to cycle reliably with most ammunition.

Pistol (required for barrels less than 10") carbine, mid, rifle. All of mine (10"- 16") are carbine. Mid is more desired for various reasons, and is popular on the 3-gun circuit and elsewhere for various performance reasons that I don't fully understand. All military barrels are carbine for the M4 and variants, barrels are usually 10" (SOCOM) or 14.5". The exceptions in the military are M16 and MK12 series rifles that are still in service.

I'm going to give this one a shot. A little less info than the link from the first post, but a little more than we've got so far. I'm not an expert, so please feel free to give any corrections you see.

The gas systems job during firing is to use enough pressurized gas from the barrel to push the BCG hard enough for the action to go through its cycle. Too little push, and its under-gassed, commonly causing short strokes. Too much push and it's over-gassed, which is harder on the rifle, rifleman, and on the brass.

At the chamber, the pressure of the gas behind the bullet is highest. As the bullet travels down the barrel the pressure gradually drops, until the bullet leaves the muzzle which allows the pressure to rapidly lower (uncork).

The gasses start venting into the gas system once the bullet passes the Gas Port. So the pressurized gas is pushing out the port, through the gas block, into the gas tube, and back against the bolt carrier key. The duration of the time when these forces are exerted (from when the bullet passes the gas port and when the bullet exits the muzzle) is called Dwell Time (when the high pressure gas is dwelling in the gas system). The force needed to cycle the action properly is a function of the Gas Pressure and the Dwell Time. (Another wrinkle is that we can increase the Port Size for any given gas system length, which will allow more gas through, but I don't want to confuse things much here.) A port closer to the chamber has higher pressure gas, so it needs less dwell time to have enough force to cycle the action. Conversely, a port farther down the barrel has lower pressure gasses, so it needs a longer dwell time to cycle the action.

On a 20" barrel with Rifle Length system, the gas starts venting into the port somewhere around 13" down the barrel. This means the dwell time is roughly during the last 7" the bullet travels through the barrel.

On the original Carbine Length systems (which I think had either 10.5" or 11.5" barrels) the gas port is a little less than 8" through the barrel. So the dwell time is only during the last 4"-ish (depending on 10.5 or 11.5) of the barrel. Because the gas port is closer to the chamber, the gas pressure is higher, so it needs less dwell time.

On the military's M-4, the 14.5" barrel worked with the Carbine gas, but not with the Rifle Length gas, so that's what Uncle Sam went with. For a civilian 16" barrel, we realized that even though the Carbine gas still worked, it was a little over gassed sometimes. So the Mid-length Gas system was born.

On a 16" Mid-Length gas system, the port is almost 10" down the barrel, so the dwell time is that last 6". Some folks find the recoil of a Mid-Length 16" AR better than a similar Carbine gassed one. Some folks prefer the aesthetics of the MidLength, because the longer handguard makes the rifle look more appealing (or the bayonet lug is at the right spot).

I think that is a rough primer on the mechanics and differences in the gas systems.

Yes, if there is a front sight tower. The gas port is a hole in the barrel that is covered by the gas block which covers and seals it. The gas tube comes out the back of the gas block.

Flat top ARs without a front sight tower just have a short gas block that is often completely covered by the hand guard, or has a short Picatinny rail segment milled into it for a flip-up front back-up sight.

Severe over-gassing can completely screw up the timing of the rifle. This was a primary reason for the problems the M16 rifle experienced when first introduced and resulted from ball powder being substituted for the powder that the rifle was designed to use and with which it qualified in military trials. This substitution was done for economic reasons and because there was an insufficient supply of cartridges with the specified powder..

The burn rate of the ball powder was such that pressure in the gas tube built up too quickly. The bolt unlocked and started to travel back before the empty case had a chance to shrink back down sufficiently for reliable extraction. The cases stuck in the chambers and jammed the weapon. The only quick remedy was to push the case out with a cleaning rod stuck down the barrel from the muzzle end.

FL-NC said:

Pistol (required for barrels less than 10") carbine, mid, rifle. All of mine (10"- 16") are carbine. Mid is more desired for various reasons, and is popular on the 3-gun circuit and elsewhere for various performance reasons that I don't fully understand. All military barrels are carbine for the M4 and variants, barrels are usually 10" (SOCOM) or 14.5". The exceptions in the military are M16 and MK12 series rifles that are still in service.

Click to expand...

USASOC is going to 14.5” CHF mid length gas barrels as a new modification program for their M4’s. Called the URG-I.

http://soldiersystems.net/2018/05/1...s-system-testing-shows-increased-performance/

Geissele will sell you one: https://geissele.com/urg-i-complete.html

I went for a full Knight’s Armament SR-15 instead which has an even longer gas system length, but the Geissele looks decent at that sale price.

The KAC is a smooth shooter, makes my Colt M4A1 SOCOM II, and Daniel Defense V11 Pro live in the safe. #KACMasterrace....

The main difference is the time between the bolt moving and time for pressure to reduce in the shell in the chamber. The short carbine length will move the bolt sooner with pressure still high in the chamber due to the shorter distance until the bullet passes the port as well as shorter distance for the gas to travel. This requires stronger extractors and generally a heavier buffer. And a cleaner rifle especially in the chamber. A mid length is better in this respect as the pressure has more time to go down in the chamber before the bolt moves resulting in less stress on parts and softer recoil . Of course Rifle length is great, as it has the longest time before the bolt moves. But there has to be enough barrel length after the port to give enough dwell time for the pressure in the gas tube to be enough to work the action. This is only an issue with rifle systems with a short barrel. I don't know about pistols. They will only function with high pressure loads and heavier bullets. Problems with port size can be helped by adjustable gas blocks. There are other factors but these are the main issues. Factory rifles usually will have properly matched parts for reliable function.

Coal Dragger said:

USASOC is going to 14.5” CHF mid length gas barrels as a new modification program for their M4’s. Called the URG-I.

http://soldiersystems.net/2018/05/1...s-system-testing-shows-increased-performance/

Geissele will sell you one: https://geissele.com/urg-i-complete.html

I went for a full Knight’s Armament SR-15 instead which has an even longer gas system length, but the Geissele looks decent at that sale price.

The KAC is a smooth shooter, makes my Colt M4A1 SOCOM II, and Daniel Defense V11 Pro live in the safe. #KACMasterrace....

Click to expand...

Thanks, that test confirms what I said. A mid length system is less stressful on parts.

Engineering can bend a person's mind every so often.
In this case (and as noted above) length = time.

So, the volume and pressure of the gas in a barrel can be predicted, and can be predicted per length.
From that prediction, the velocity through the gas port can also be predicted.
Which can be converted to time for knowing the length of the gas port (the barrel thickness and the height of the gas block).
The velocity/pressure graph for when the gas enters the gas tube can also be predicted.

Timing is a metric, as we can predict how long it will take the bullet to go from the gas port to the muzzle. We want the length (time) of the gas tube to exceed (by some amount) the time taken for the bullet to clear the muzzle.

Normally, you only have to design that once.

However, if you want to change the barrel length, then the formulas change. Now, they are all linked together, so there is some "play" in how it works. So, Lgp and Lm are linked to Lgt. Ok, we want to reduce Lgp with a reduced length barrel. One "fix" is to change Lm. The other is to change Lgt.

As an example of that, some of the earliest pistol ARs used a "corkscrew" gas tube. That added length to the gas tube without changing the distance from one end to the other. Another way is to change the diameter of the gas tube, as that changes the velocity (time) for the gas passage.