Actually the XCR is an amalgamation of three rifles; the AR, AK, and FAL. In terms of the action, it is by far more closely related to the AK than the AR or FAL. It is *not* a gas piston AR15, and shares no major component compatibility with the AR. It uses a long stroke piston with the recoil spring coiled up in the piston, and a carrier and three lug rotating bolt that operate very similarly to the AK. The FAL (and as I understand it, many of the AR15 piston additions out there) use a short stroke piston where the piston is not attached to the carrier.
The gas plug/regulator is roughly FAL-like, though these two parts have been combined into one adjustable gas plug on the XCR. The bolt catch is located in the same place as the FAL's, but is closer to the trigger and ambidextrous, making it much more easy to operate. The charging handle is on the left side, identical to the FAL.
The AR influence is actally rather limited (mostly to the ergonomics); the selector is ergonomically similar, but rotates 45 degrees rather than 90. It takes AR pistol grips, and uses a similar mag catch. It also uses AR15 style magazines. Other than that, the lowers are different.
The upper is monolithic and the barrel is free floated/QD, somewhat similar in concept to the LMT MRP.
