so i warmed up to the AR15 platform. as expected, availability was slim, and even though i had tried to research before and during my hunt for the best possible rifle the few rifles i could afford were not what i had seen online. well i bought a Sig M400 thread, 5.56, seems to be everything i need. the 5.56 is what i wanted, but i am curious about some larger calibers, 300blk, 6.5 Grendel, maybe a 45type option. what i am wondering about is whether anything but Sig uppers will fit? i am guessing i could buy something off PSA or most any other builder offers, but i have guessed wrong occasionally in the past. is Sig a true AR compatible platform or is it proprietary in any respect. I had looked at an entry level Ruger, forget the model, but i was disappointed in the finish so i kept looking. then i found the Sig at a LGS that i like. i am pretty happy with the fit and finish of it, seems to be Sig Sauer grade quality... anyone else own a Sig M400? make any modifications ?