I am going to buy a HK piston AR with a 10.4" barrel just like the HK416. The 2 routes I know to go are:
1. Buy the HK MR556 rifle and get the 16.5" barrel cut down to 10.4 inches, re-profiled and rethreaded by a qualified gunsmith.(IGF) Mod or replace the gas block with the HK 416 gas block...