There are a number of semi-auto platforms out there with a large number of them using either the Direct Impingement system or the Piston-Driven system. The one that really seems to garner the most controversy is the AR-15 system. The Direct Impingement system was the original design.
The AR-15‘s gas system uses a gas port in its barrel that siphons away a portion of the gas expended when a round is fired. The gas travels through the gas block (usually underneath the front sight assembly) and down the gas tube until it reaches the bolt carrier group (BCG). The gas tube is a hollow metal tube that runs above the barrel down the length of the barrel. Gas reaches the gas key that’s mounted on top of the bolt carrier group. When the gas hits the gas key, the bolt is unlocked, pushed to the rear, and the spent casing from the round is ejected. Overall, it’s a reliable system.
Comparison of piston-driven (top) and direct impingement (bottom) AR-15s. The gas tube on the bottom AR-15 extends from the rear sight along the top of the barrel. Each system has its drawbacks and advantages. Photo by Modern Firearms.net.
Drawbacks Of Direct Impingement
Opponents of this system say the gas that opens the bolt carries carbon, unburnt powder, and other nasties directly into the rifle’s action and chamber. They also point out that the heat from the gas causes the action of the rifle to heat up. Some claim that this drastically reduces the reliability of the AR-15/M-16 system.
Is it really as bad as that, though? I think not. The system’s worked since designed in the 1950’s. Yes, there were issues early on in Vietnam. However, the bullet’s propellant was changed and it didn’t agree with the rifles. Additionally, troops were told the rifle was “self-cleaning” and they weren’t issued cleaning kits for their rifles! This obviously was not the case. We now know all weapons need to be cleaned at some point.