Can someone explain to me the difference between colt / oly uppers? I think I want to go with OLY and just buy a mag block right? I don't know jack shit about them yet except im going to order a lower this morning. Just to clarify, it dosnt matter which kit I use I can always go back to 556 right? And just register the lower as multiple calibers and barrel lengths from 5" - 16" and cal .17 - .50 ?