I don't want to start a fight....but I have access to a Colt ar 15.
It's barrel is a 1:7 twist i belive It might be a 1:9, either of the two are the standard twist rate for the ar15 platform, and is accurate as all hell. As for different pin sizes etc, I have never had a problem swapping uppers with the Colt parts, nor replacing the inners with any parts kit available.
For headspacing I took mine to a local gun smith, and 30 minutes later it was all set, and cost me 30 bucks to check...soo that wasn't an issue.
He's asking about a AR15, I gave him a suggestion. If he was looking for something in a larger caliber, i would have suggested something else.
The .223 / 5.56 round is more than enough to stop someone in the 200-300 yard range. It's only longer than that were you need more umph, and the .308 shines.
I've shot a m1a/m14/m21 and m1 garand. the recoil of these fine firearms is a bit more than .223/5.56.
The BEST THING about an ar15 in .223....is you can easily convert the gun to shoot .22lr, which makes sighting, and practicing cheaper, while allowing you to actually shoot it often.
if they stop the sales of semi-autos, they can not and will not go after what people already own, and that's if it actually happens....chances are, it wont. If it does, it will probably be in the form of an assult weapons ban again, anything more will be met with great resistance.
Bushmasters are good, RRA is another great company.
However, Colt made it famous, and its colt that will always be the Best IMO