What you want is an M-4 clone with the 16" barrel, flattop and possibly the rail system handguards. The latter is dependent upon how custom you want to go. You may as well get a Colt. There are lots of good brands that will do you just as well as Colt, but why not? I like a simple gun. Right now mine has iron sights and an EOTech type holo-sight. You may want a scope.
I'd get a second upper in 6.8 for hawgs, as the others have said. Or just get two guns.
Get the Magpul mags for extras. They're the best.
I've owned probably half-a-dozen Colts since 1982 and three Olympics. I've had no complaints about any except a few FTE's from a current M-4 I've got. That's a pretty good record though.
