FNG here, but I'll try to keep this relatively short. Purchased a complete upper in .300 Blackout at a local (DFW) gun show this past fall from Durkin Tactical. I then built a complete lower using an Aero Precision lower, Velocity drop-in trigger, and assorted other parts. Between that time and Christmas, I saw an Aero Precision stripped upper for a good price and decided that I'd like to have matching receivers. I asked for (and got) the AP upper for Christmas, along with a 10.5" M-Lok hand-guard to replace my 15" keymod, and excitedly took all this to my local 'smith for swapping (since I don't have all the needed tools and gauges). His fee seemed fair, but when he was done he said that I should contact the barrel manufacturer as the headspace appeared to be off. Using the original barrel, bolt and BCG, he could close the bolt on the "go" gauge, but only using light tapping with a mallet, not by hand. And again, the mallet was required to remove the bolt. He did try different bolts too (he said). I brought the rifle home and contacted Durkin Tactical. They assure me that every upper is checked before being sold, and that the issue must be related to something else (out-of-spec receiver, improper barrel torque, or some other 'smithing error). So... I've visited two other local shops to try and get a second opinion on the headspace issue, only to discover that one shop doesn't have a 'smith anymore (despite that being advertised on their website still), and another that didn't have the gauges for .300 Blackout (although they could check .223 all day long). Anyone here live in the DFW area (particularly Plano/Richardson area) that might be able to help me? I'd really like to shoot this gun one day, but I am determined to ensure it is safe first.