I just bought myself a Sig M400 yesterday for my birthday. So far I like the look and feel of it, and it came standard with a few extras. It has a spring tension pin to keep the upper and lower tight, quick detach holes built in the lower receiver, ambidextrous mag release, detachable handle, cold hammer forged barrel, and some sort of grip on the front of the mag well. I put on a Daniel's Defense omega, magpul angled fore grip, and eotech, and I think later I'll switch the stock and grip as well. The trigger is a single stage(I like my RRA 2 stage trigger better), M-4 feed ramp, and barrel is chrome lined. I'll have a range report soon, hopefully a good one. If all turns out well, I might try Sig's 516 piston AR.