Like I’ve said before, because there are so many different versions of AR-15 rifles out there, a lot of guys don’t really know what they have. They know they have an “AR-15″ or an “M4″, but those are pretty generic terms these days. One of the things I run into in my business is people ordering Picatinny Rail Forearms that don’t fit their gun. One of the most basic things you need to know about your AR-15, or the one you want to buy is what length gas system it has.
“Gas System”, you say, “This is a gun not a car!” Well, AR-15s are “gas operated”! They use the gases released when the bullet is fired to cycle the action. There is a tiny hole drilled in the top of the barrel that allows the gas to vent up into the gas block, into the gas tube, and then blasting back into the gas key on top of the bolt carrier thereby pushing it backward to eject the spent shell casing. OK, that’s about as technical as I’m going to get! (And by the way, when you buy those cheap parts kits out of Shotgun News, etc, the gas hole in the barrel is typically screwed up. Those cheap kits are made up from factory rejects!)
There are basically three different length gas systems; Carbine, Midlength, and Rifle. Carbine is found on 14.7″ and 16″ barrels. Midlength is also found on 16″ barrels (and 18″). See why there can be some confusion. Rifle length in found on 20″ and longer barrels. The real confusion is on guns with 16″ barrels. Most have Carbine length gas systems, but some have Midlength.
On the picture above, you can see the basic differences. Carbine uses forearms about 7 inches long, Midlength uses forearms about 9 inches long, and Rifle uses 12 inch forearms. If you want to switch out the forearm, you need to know what length you need. Now it doesn’t matter what Brand AR-15 you have. What matters is the length of your gas system.
So what’s the difference? Why do they even make Midlength? Well, look at the picture above, but this time look at the distance from the front sight to the muzzle end of the barrel. You’ll notice the Carbine barrel is much longer from the “Gas Hole” to the end of the barrel where the bullet exits. In fact, its about 2 inches longer. Midlength and Rifle are about the same. Once the bullet is fired, and passes the gas hole, it begins pushing gas up into the gas tube and back into the gas key on the bolt carrier. The longer this happens, the more blast and force there is on the gas key and therefore more of a “pop” of recoil. The bullet is in a Carbine barrel for 2 inches longer than the other barrels, therefore pushing more gas back into the action. This is why Carbines seem to have a sharper recoil, not that the .223 produces much recoil, than Midlength or Rifle barrels.
If you fire them side by side, you will feel the difference. A 16 inch Midlength gas system is smoother than the exact same rifle with a Carbine gas system.
Does it really matter? It’s really a matter of personal preference. It’s not a big deal to me. To some people it is. Some people will only own Midlength guns. Here’s an interesting parting note… 14.7 inch barrels, with fixed flash suppressors, are about the same length from Gas Hole to muzzle as Midlength and Rifle systems and thereby give you the same smooth recoil in the Carbine length gas system. Hopefully that doesn’t utterly confuse anyone!