As to artillery of the period discussed, wasn't most of it cast similar to a bell and the barrel drilled out to its final dimensions? If I have the correct type of gun in mind, there wouldn't be a plug, just the smooth cast exterior at the breech/touchhole end.

Sounds about right!

private entrance,
if that trailer ball has a cap on it it means the boltgoes all the way through the ball. in other word its iin two peices. when you tighten the nut you have to be real careful that you dont squash th ball. basically they are junk as far as towing goes. i think you will find the ball itself pretty soft. they are hollow.

Was afraid of that. I may have to just split it open and use the metal on it for something else.

Trailer balls are GREAT bottom tools as they are for forming things round. Use something else for your hardy.....