If Ruger makes just the upper available for a decent price, I'd consider giving one a whirl. I am not as hung up on names when it comes to AR's as some of the others here. My first issued rifle was a Colt, the second was made by G.M. The third through sixth were made by F.N., and then I bought a Bushmaster. Now I have a Colt again. I never saw any kind of difference in the functionality or reliability between any of them, although the first Colt did have the worst finish of them all but then a lot has happened with finishes since the '70's.

