Don't use the drill end
Measure your breech plug flash hole and record it
through out the year occasionally re-measure the Flash hole
Replace Breech Plug as necessary
Hope this helps

They do make pin gages that you can use but the grip end of a drill bit works for this purpose. Just FYI if your flash hole is actually say .030" you won't get a .030" bit in there. Likely the smallest you'll get in is a .029" or maybe if both are perfectly round a .0295".

Yes but a #69 (.292) and #68 (.031) is probably within specs of the Breech Plug Flash Hole. Personally I don't know of any gun that that is not true for, But I don't own them all. Thanks for the info, I do use pin Gauge ZZ Minus for checking bore diameter but I think the pin gauges would cost a lot more for a set to cover most Breech Plugs.

D it sounds to me that like me you have either a machinists or inspection background. I was the latter for about 45 years before retiring.