I'm building a pistol that will eventually be an SBR once my Form 1 comes back. My lower came in yesterday and I finally got a chance to test fit the upper and lower. So either my Noveske lower is out of spec or my VLTOR upper is out of spec. They're so tight that I can't fit the rear takedown pin in without a mallet. The upper is tight but workable with my two Eagle Arms lowers, but fits perfectly on my friend's PSA lower. My RRA upper fits fine on the Noveske Lower. When I squeeze the VLTOR upper on the Noveske lower I can see the bottom of the inside of the upper's rear takedown pin hole sticking up into the lower's pin hole. It's like the log doesn't hang down far enough, or the hole on the lug is a couple thousandths off. I emailed VLTOR and they gave me a RMA to send it back for them to check to see if it's in spec, but in a stroke of frustration last night I decided to just clearance the rear pin bit grinding a little bit of off the bottom (figuring I can get a new pin if I want for a couple dollars. Now I have an out of spec pin that makes the upper and lower fit together like a glove. With that out of the way, I spent the rest of the evening installing the barrel and rail. Now that I've got the upper assembled, I'm re-thinking sending it back to VLTOR to get it checked out. My fear is that with the pin ground down and metal exposed, that I'm opening myself up to rust on that pin. I want to get my Form 1 sent off, so I don't really feel like sending the lower back to get checked out if I have a viable work-around. My expectation is that neither the lower of the upper are "out of spec", but more along the lines of having opposing tolerances that stack to cause the problem. Would I be better off just sticking with a slightly modified rear takedown pin? Any suggestions on how to treat the exposed metal on that pin to keep it from rusting? VLTOR MUR-1(a?) upper Noveske N4 Gen II Lower Noveske 8.2" 300BLK barrel Daniel Defense Lite 7.0 Rail AAC 51T Muzzle Brake (goes with my AAC 762-SDN-6 that's pending) VLTOR BCM Gunfighter Charging Handle PSA Pistol Buffer Tube PSA LPK minus FCG (not installed) No LPK installed, BCG, or buffer in place. I'm not going to install the LPK until after the lower gets engraved and it'll be next month before I can buy the BCG and SSA-E trigger. Shown with a mag full of 5.56 for context.