My guess is the 'back flash' you are experiencing is not happening because the flash hole is getting plugged up. It is probably due to the flash channel getting plugged up. The flash channel has a diameter of 1/8". Chances are you cannot get a 1/8" drill into your flash channel, because it is partially plugged with a very hard carbon. This is probably what is causing your 'back flash'.

What you should do is start with a smaller drill, and spin it by hand in your plugged flash channel, and move to larger drills until you can hand spin the 1/8" drill. Then you should be able to shoot your rifle without the 'back flash'.

The flash hole will take a #70 drill, and maybe even a #69 drill. If it has worn, and eroded, it will accept a #68 drill. However, you don't need to use any of these drill in the flash hole.

Keep the flash channel some open. Perhaps use the 1/8" drill every 15 or 20 shots or so. What ever it takes to keep the 'flash back' from getting too bad.

If you have experienced enough of the 'flash back' to flame cut the primer seat, you won't be able to stop the 'flash back' with a clean flash channel. You will need to repair the damage, or purchase a new breech plug.