Most AR-15s use a gas impingement system to cycle. This means that gases go through the port of the barrel, through a gas tube into a chamber in the bolt carrier where the gases push the bolt forward, cycling the bolt carrier group backward. This type of system makes for a very lightweight firearm.
However, the gases are dirty and foul the inside of the upper receiver and the bolt, making the system less reliable and requiring more cleaning and maintenance. This system also puts a lot of gases into the face of the shooter when he is using a suppressor.
The XCR is piston-driven with a gas block a certain distance down the barrel from the chamber. The gas block directs the gases against a piston which cycles the action. Because the piston is located down the barrel a distance, the action does not get fouled (dirty) as fast. Piston-driven rifles are simply more reliable and require less maintenance.
Many incorrectly assume, that the piston drive and loose tolerances are what make the AK-47 such a reliable weapon. This is only partially correct. The AK-47 has tight tolerances where needed. The AK-47 also feeds better, cycles smoother, extracts, and ejects better because of other parts of its design (which, by the way, are incorporated into the XCR).
Many have added small pistons-driven systems to their AR-15s but these systems cannot be optimized for the lack of room, and don’t have the other features that make the AK-47 and XCR so reliable.
Not only is the XCR gas piston driven, but it’s adjustable with 7 discreet settings. This allows the operator to maximize reliability and minimize recoil with almost any ammo.The XCR’s gas adjustment system is a must-have when using suppressors. Some suppressors are very quiet but have tons of back pressure which cycles the action extra hard. Having 7 settings helps a lot in taming the recoil of some suppressors.