AR-15 Build question

flybynyt · May 19, 2010

OK, to those that know this will seem dumb...so here goes..

I am building my first AR-15 pistol, and I don't really understand the gas system.
In other words, I get that the piston in the buffer tube retracts and then expands with the bolt, but how does the gas block, tube etc..come into play?

I can't figure out how it all works together.

Also I see that different company's sell buffer tube assembly's for AR pistols, that are very different in length. Will one size work better than another?

Thanks
F

NoAim · May 19, 2010

flybynyt said:
OK, to those that know this will seem dumb...so here goes..

I am building my first AR-15 pistol, and I don't really understand the gas system.
In other words, I get that the piston in the buffer tube retracts and then expands with the bolt, but how does the gas block, tube etc..come into play?

I can't figure out how it all works together.

Also I see that different company's sell buffer tube assembly's for AR pistols, that are very different in length. Will one size work better than another?

Thanks
F

So let's start fully loaded and ready to fire. You pull the trigger and then:
1) Bullet flies down the barrel with the high pressure gas behind it (burning gun powder).
2) About 4-6" from the muzzle (depending on barrel length) there is a small hole in the barrel called the gas port. Some of that high pressure gas behind the bullet is siphoned off.
3) This gas then travels down the gas tube back into the carrier key of the bolt carrier in the upper receiver.
4) Where it empties into is a closed area between the carrier and the gas rings around the bolt. This gas wants to expand. Since the bolt is locked in the barrel extension, the only thing that can move is the carrier. So the carrier starts moving rearward.
5) As the carrier moves rearward, the interaction of the cam pin and bolt cause the bolt to rotate to unlocking.
6) Once the bolt is fully unlocked and extended, the gas ports on the side of the carrier are exposed and the rest of the gas is vented out the ejection port.
7) The energy and momentum imparted to the carrier group makes it continue to travel rearwards against the buffer weight and spring in the buffer tube.
8) The whole assembly travels rearward and bumps up against the rear of the buffer tube. During this travel the hammer is reset against the disconnector and the bolt face has cleared the ejection port and the spent case is ejected.
9) With the compressed spring, the carrier group is forced forward again. Along the way the bolt strips another round out of the mag. Everything slams forward into the barrel extension and it's ready to fire again.

So the short answer is that attached to the barrel is a gas block. This is lined up with the gas port in the barrel. The gas tube carries some siphoned gas into the bolt carrier which forces it to unlock the bolt.

Note that this is for your standard Direct Impingement AR-15 and not the new-fangled piston drive ones. In a standard AR-15 there is no real piston. Instead the bolt carrier acts like one.