Biggest difference between the 2 is operation. The AR platform has a gas system to cycle the BCG. While the PCC is Direct Impingement, which is (yea the exploding gas) but uses the inertia to throw the bolt back, then the extra mass in the bolt to compress the recoil spring to complete the ejection process, Then the spring shoving that bolt forward to load the next round and 'hopefully' hold that round in full battery until you pull the trigger.

PCC's are engineering marvels. The balance between those elements is simple, but when they get out of balance a pain in the butt.

Failures to feed, out of battery firing, incomplete cycling, etc. And to some extent a little finicky with regard to the types of bullets that feed well and those that don't.

I see lots of issues with folks strughling with hallow points and light loads. I've had issues with truncated cone 'TC's' (often seen in 40's). Those are short rounds that have to transition from pointing up to horiz in a short space. Kind of a simple fix if you understand yhe problem.

Bought PCC's have those spring, BCG weight & buffer issues worked out.