I own a PWS MK114. It's a long stroke piston system (think AK47) but in an AR15 platform. It's NJ compliant from the factory as far as the muzzle break goes (14.5" barrel and PWS FSC556 compensator). I had the dealer pin the stock for me and swap out the 30 round mag for a 15 round however. Under $1500, quad rail, Magpul furniture and sights, BCM gunfighter charging handle, enhanced buffer tube, FSC556 comp, 5 QD mounts and it weighs a hair over 6.5 pounds! And she runs so cool and clean you can shoot 100 rounds and then swap out a bolt bare handed no heat or fouling gets transferred to the bcg.
Proprietary? Yes, yes quite a bit. So I bought a complete extra bcg and bolt as spare. No bigs. It shoots great and is a flippin breeze to maintain as compared to my old Stag from back in the day.
The best part at least for me is that as a left handed shooter I get hardly any gases expelled in my face as compared to a DI rifle.