I know you want a tight upper to barrel connection, but is there too tight?

[Hoser1]

When I put my AA side charger and JP barrel together, I used a dead blow hammer to get mine seated. It did start into the upper okay, then got gradually tighter as it was being seated. Have you put a micrometer to see what the difference is? you stated that it was a "Blem", is it possible that all the machining wasn't complete? Maybe "truing the upper" might be enough to get it to seat. Hopefully some more experienced guns builders will chime in. Get yourself an upper holding block, that will help in your build, not only to seat the barrel but to get the barrel nut torqued.