I had issues with duracoating an AR receiver and using it with a tight fitting upper like the vltor MUR. I kept my coats thin on the top of the receiver, yet it was still enough to make aligning the takedown pins impossible.
To correct, I got some sandpaper of varying grits and lapped the top of the reciever by what I would expect the coating thickness to be. Recoated and it was fixed. Also had the same issue with the very top of the receiver where the buffer tube connects. Had to lap it so that everytime I pulled back on the charging handle it wasnt digging into the finish.
Its trial in error, but with a looser fitting upper, I may not have had an issue.
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