I have a Robinson Arms XCR in 5.56
Very interesting solid built modular gun. Took the rails and feature of the AR and used the piston and bolt setup from an AK. Extremely simple, very rugged and the controls are by far the best (maybe as perfect as they get).
You can change calibers to 6.8 and 7.72 x 39 with a barrel and bolt change (barrel only I think in 6.8). I have taken the barrel off and back on and no change in point of impact. You can get to everything to clean or maintain on this gun easily (barrel and piston come off and back on dead easy)
PM me if you want to talk as mine is for sale. I am more into target shooting and got a tack driver AR that does 5 shot groups good. I hate the AR controls, but its sub MOA accuracy. Also inherited an AK and getting too many guns to shoot as often as I want (and loured into 1903s) .
The XCR was good, shot some great 3 shot groups. After that it was more 1.25 MOA.
Stock folds like an AK as well. Truly an amazing gun that would make the excellent basis for a new military arms for the troops. Better then the ACR and the SCAR in my opinion.