Helllllllp! I want to build a rifle using a Colt lower. Will a current production Colt lower fit most uppers and which is the most common, big hole or small hole? Is it just the uppers that are  different? Does anyone know of a webpage that explains this screwy mess of a phenomenom in detail?

Current production Colt is small hole for the upper and lower assemblies. This means you can use any Bushmaster-RRA-Armalite-whatever upper on a current Colt lower.

Colt prebans were 99% large hole. Then came the switch over about the same time as the Blue label guns where you will find some small hole guns and some large hole guns in that line/time frame.

[url=http://www.biggerhammer.net/ar15/]Biggerhammer.net[/url] has a lot of good info.

Just as with everything else a SMALL hole is always better LOL couldnt resist