Can you swap a 6.5 upper with a 5.56 and interchange them anytime you please?

[wiuprof]

Yes, what mlmiller1 said. I have had this Colt HBAR Match (heavy barrel) since about 1990 or 1991 and it has a larger diameter front pin. I bought the double diameter pin that's pictured so as to allow the use of all the other uppers out there. It got to be a pain in the butt, so I just wound up getting a couple of other AR's to more easily swap out uppers. It wasn't that big a deal, but it was a reason to get some more rifles!

I still have the Colt, of course.