At sometime in the pass someone got something stuck in the barrel to cause the bulge. Look at rib post #4 and you can see where it lifted up. This is not from the factory.
Can you shoot it, probably,, could something go wrong and it blow up, probably, would I shoot it,, probably not.. Look for...
Do yourself a favor and have someone that knows what there doing do.it,,, all you will do is screw it up, you will never be able to do it with sandpaper or anything else you mentioned.
Shotshell psi is over 11500 psi, closer to 12500 max, and the barrels are tested a lot higher then that. I'm...
When I built mind I did a lot of research first,, Bought the upper complete from SOTA Arms with the SS barrel,,, built the lower with Gen 1 ODT lower,
Biggest problem with the uppers weather you build them or buy complete is 1.. the mags not feeding right,, 2.. the bolt breaking like others...