My Rguns upper finally came in the mail today. By all accounts it is a beautiful piece of machinery. When I tried attaching it to my lower the front was a little snug but went right into place. However, the rear is just too tight to fit. I remember being told this is a somewhat common issue. I'm just curious about two things... 1) What do I want to take material off of...the lower receiver (channel) or the upper receiver (male end that fits into channel)? and 2) What tool is best for the job? I've got everything from paper to files to a dremmel.... Thanks! -Emt1581 Wanna kill these ads? We can help!