This flame cutting i have experienced more than once, seems to be rare, but i certainly have seen it in at least 3 breech plugs of mine. Others don't seem to experience the issue.

The information you provided has enabled me to install a grade 8 bolt in two breech plugs with flame cut primer seats. One was an Accura plug, and the other an Omega plug. They both also have vent liners installed. These plug are now better than 'new'. For the past two days, i have been utilizing the newly rebuilt Accura plug . The only problem so far is removing the W209 primer. It seems the letter C drill makes a primer pocket some tighter than factory. The W209 primer has a groove running around it that the Federal 209A doesn't. I believe this groove makes for difficult extraction of the W209 every now and again.

Using the grade 8 bolt has allowed me to 'salvage' breech plugs, that would otherwise have been deposited in the trash. Another advantage is, i was able to custom fit the depth of the primer pocket to my rifles. The Omega breech plug now allows zero blow back. The Accura breech plug blackens the primers some, but blow back is very minimal. Reliability so far is 100% using BH209 powder and W209 primer, with both breech plugs.

Without your information, my enjoyment of my rifles would be some less.