I have broke corners with a small flat Jeweller's File and then polished the area with a Dremel and a Felt Bob using JB Bore Paste.
In fact, I do it as a matter of routine on a new barrel install for insurance. Cleaning the stray JB out of the barrel extension is the biggest hassle.
It can be done with the barrel in the reciever but it is easier to do with it out of the upper.
It is hard to over-do-it with such limited tools.
My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.
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