Screwbolts

Thank you for your reply and suggestions. I agree that the 'groove' in the primer seat is from flame cutting. After i made my post i examined the breech plug more closely, and saw evidence that there was a hole being bored sideways through the breech plug by the 'flame'. The flame cutting in that area may have occurred because of a milling defect, but now it is definitely being cut deeper and broader by 'flame'.

My brief experience modifying breech plugs, has shown me that the material of the CVA breech plug is definitely 'softer' than the material of the TC breech plug.

Another question that entered my brain is if drilling the flame channel all the way through the primer pocket is a mistake. I am wondering if when one drills through the flash hole with the #21 drill in preparation of tapping for the vent liner, if one shouldn't stop short of going all the way through the end of the breech plug. This would leave a broader 'ledge' for the end of the primer to sit on, and perhaps help reduce blow back and thus the 'flame cutting' of the seat.

I just spent a little time searching for the 'C' drill, and found there are two types commonly available. There are 118* and 135* drills. If i correctly understand the nomenclature, what i should purchase is the 135* C drill, which i believe would be the 'flattest'. However, this doesn't seem flat enough to mill a primer pocket. After purchasing these drill, will i then need to grind the tips flatter to make a more better primer pocket?