Currently building an ar-15 SBR (legally w/tax stamp). Anyways upon assembly I noticed the BCG fits very snug in the upper receiver. I can push it all the way in until the lugs lockup in the chamber. But it will not slide freely like it does in my other ar's. So my question is more or less can I take some sort of abrasive and polish down the lower edges of the bcg (which is where I believe it is catching) or should I fully complete the rifle, lube it up good and just work the action (chamber empty) to break it in a little. I have never had this happen in my other ar builds, so this is a new one on me.