I bought a used Colt 20" HBAR (MT6601). Its in very good shape from what i can tell. The only thing is when I cleaned it I noticed the bolt doesn't slide too easily in and out of the bolt carrier. In other words, when you hold the assembled bolt carrier in your hand and shake it, the bolt doesnt easily extend. It will if you shake it but not like I see them freely move in some of the cleaning videos I've seen. The extractor pin is properly in place as well as cam pin etc. The friction seems to be on the slightly raised ring on the bolt rubbing against the inside of the carrier. I know w knew guns there can be a "break in" period where this happens but this is a used gun. Haven't shot it yet and the guy I bought it from is out of the country for some time so I can't ask him. Any thoughts?