The simplified difference between AR-15 gas systems

| 28 September 2014 | 0 Comments

The simplified difference between Carbine, Mid and Rifle length gas systems on an AR15’s

With the increased number of AR-15 rifles coming into the country and been used in our sporting disciplines, coupled with the relative ease of sourcing one,  there has been a keen interest by many who may not have before considered buying one.

Invariably in the discussion of which rifle to buy, the question of which “gas length” will too come up.

So what exactly is a “gas length” or “gas system”?

It’s a surprisingly simple concept, but a system that is not always that easy to set up correctly, as some manufactures have found out.

The system uses some of the high pressure gas generated when firing a cartridge. This it directs through a small hole and then through the gas block mounted on the barrel. The gas then travels back down a tube into the receiver where it forces the bolt backwards extracting the empty case. The bolt on the return, will then chamber a new cartridge. This system is known as a Direct Impingement Gas System.

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What are the different lengths and why does the length of the gas system matter?

The length of the gas system is determined by the distance between the gas port and the receiver. Generally this increases with the length of the barrel. The reason for the increase in length has to do with what is termed “dwell time”. That is the length of time that the bullet is still in the barrel after the shot has been fired.

On longer barrels, the dwell time will obviously be longer as the bullet travels a longer distance.

Dwell time after the gas port is affected by two things, the placement of the gas port and the length of barrel. It’s during this last part of the dwell time, that the hot gases are forced up through the gas port, down the tube and into the receiver. The longer this dwell time the more gas is forced into the system.  Allowing too much gas to flow into the receiver, can causes issues with excessive recoil and wear, too little and the rifle will not cycle correctly. As soon as the bullet exits the muzzle, the gas stops flowing into the system.

If you break down the three different lengths you will find that the carbine system is found on 7″, 14.5″ and 16″ barrels.  Mid length systems are also found on 16″ barrels and on some 18″ systems (Yes it is confusing) and rifle length systems are found on 20″ and longer barrels.

Rifles with 16″ barrels can be particularly confusing as generally they have Carbine length gas systems, but there are some manufacturers that use mid length systems.

On the first two barrels illustrated below, you will see that the distance between the gas hole and the muzzle is quite a bit longer on the carbine length system. This means a slightly longer dwell time past the gas port and therefore more gas into the system. For this reason a 16” mid length gas system is smoother than a 16” carbine system.

There are of course a number of other factors which can influence the gas system, which includes the type of ammunition you use, adjustable gas blocks and different buffer weights. The strength of the buffer springs will also have an influence on the system.

This is a simplification of the system, but I hope that it was helpful in some way.




Category: Guns & Gear

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