My first choice would be a Colt 6920. The 1:7 barrel gets my vote. It's a big bite, but once you take it, you're done.

If I couldn't pull that off, I'd get the nearest Bushmaster, RRA, Armalite or Stag clone to the above, provided it had the chrome-lined barrel and bolt carrier, at the very minimum.

After shooting a couple thousand rounds of various ammunition, to make sure I had a reliable shooter, well then...

I'd build me the most uber tactical, ultra-high spaceage technology, super-sweet shooting implement of destruction the world has ever seen.