Mil-Spec is a meaningless buzz-word.
NO Bushmaster, Colt, or ANY OTHER AR-15 is truly 100% "mil-spec", but some people just don't seem to get it. If you sold any AR-15 to any military in the world they would be made MAD. Flame away, boys...
Which AR?
I suggest Colt, Armalite, or Bushmaster (in no particular order), 16-20 inch barrel only, 1-9 twist, A1 upper is accurate and durable, an A2 is very accurate but no as durable, and an A3 upper will give you the option of putting on some really nice optics, and don't let anybody tell you you NEED to put a brake/comp on it- if you like the way they look , fine, but don't buy any claim that the muzzle MUST be protected by one. Might as well put an AK brake on that .30-06 deer rifle...
Welcome, and happy shooting!