You hit the nail on the head !!Not long ago, I bought a new, old stock Traditions Tennessee rifle.
The clean out screw on mine is stuck....
For me it's a nonissue. I always remove the nipple when I clean the fire channel and use pipe cleaners to get in there.
Leave that screw alone and run a pipe cleaner down thru the flame channel from the nipple hole to the breech after the nipple is removed.
Safe and simple and no chance of boogering up the little plug screw slot.
People who own one of the later made Thompson Center sidelock guns don't have an option. Thompson Center stopped drilling the flame channel from the outside of the barrel so they don't even have that stupid screw.