Modular and customizable are two of the most common descriptors when it comes to talking about AR 15s, to the point that they almost feel like marketing buzzwords. But the truth is, talking about the AR’s customizability and modular nature so frequently makes sense because that is, pretty much across the board, what owners love so much about this subset of firearm. If you decide one day that you want a shorter barrel, it’s easy enough to find an upper parts kit or purchase a complete upper assembly to swap out for the function, design, and components you want. In fact, many AR owners choose to have a few different upper assembly options to swap between when using their AR for different things (i.e. firing at a range versus hunting).
The modular nature of the AR 15 means you can change practically every component of the upper assembly from the handguard rail to the direct impingement system. What you choose depends on your preferences and the functionality you seek. One of the big points of discussion in the AR community is the debate over whether a direct impingement system is more or less effective than a piston system for firing, accuracy, and long-term use. Is one system better than the other?
When it comes to the firing system for AR 15s, there are two options; you can choose either direct impingement or an internal piston. First, we ought to clarify something: Direct impingement is a bit of a misnomer. For the gas-powered system to truly be direct impingement, firing would be done with only the force of gas pressure, rather like a hammer made of air-pressure. Instead, what AR owners call direct impingement is using that gas pressure to operate the bolt carrier group, fire a cartridge, redirect the gas back down to the gas block, and eject the spent round and chamber a new one, making the firearm ready to fire again nearly instantly.
Piston operation still uses gas to operate the bolt carrier group, but not in the same way direct impingement does. With piston operation, gas moves down the barrel driving the bullet forward; however, instead of going into the gas block and being redirected back, the gas is directed up into a cylinder where some of that pressure is used to push a piston forward and reset the bolt carrier group. The excess gas is simply vented out of the rifle.
Which is Better?
As with so many of the best AR accessories and components, the delineation of “better” is subjective and largely based on your own preferences. However, there are a few differences in the two systems that may help you choose which will suit you better.
Putting on the Heat
The first really noticeable difference between the two systems is temperature. With a direct impingement system, the gas heats up quickly and, since it travels through the upper receiver, most of the upper assembly will heat up quickly. It doesn’t matter much unless you go to pull apart the upper assembly right after firing a few magazines, but many of the best AR 15 handguard rail systems double as heat protection if you want that added protection. On the flip side, because most of the gas vents off with a piston system, the upper assembly won’t heat nearly as much. All this means is, if you choose direct impingement, you’ll need to choose a temperature-resistant gun oil.
The primary point of a direct impingement system is to make firing as smooth as possible. The difference in the two firing systems isn’t huge, but a piston system moves less linearly and has more mass than a direct impingement system, which can (usually minorly) affect firing.
Finally, the other big difference between the two systems is one of dirtiness. A common phrase amongst the AR community is that direct impingement is like going to the bathroom where you eat; with direct impingement, carbon-filled gas is carried back to the bolt area. However, the carbon that gets carried back to the bolt area is negligible at best; as long as you keep up with regular cleaning and maintenance, that extra bit of dirtiness won’t affect your AR’s operation or cause it to malfunction.