These are photos taken off a TV screen of a bore scope video. My diesel mechanic son joined the USMC and left his Snap On bore scope behind only has a 36" probe though.
This is a 38" 50 caliber barrel breeched by the maker.
The rifle maker then bungled the vent liner (and almost everything else) it protrudes into the bore.
The breech plug forms a wonderful fouling trap that makes it impossible to get the fouling out. It and the screw slot equipped liner have been out and cleaned at this point the rebate on the breech plug was totally packed with fouling but no rust, it had been oiled and it lives in low humidity. BP fouling is virtually non-corrosive below 30% RH.
The face of the breech is smaller than the land diameter and looking into grooves you can see about .075"-.1" back to the threads which do not reach the end of the drilled hole (?) in the breech.
If shot with something really aggressive like some of the "replica" powders this thing will probably fail by the fouling eating out to the side of the barrel.
As is it is difficult to clean and makes an "oil trap" that can contaminate the powder charge.
The other barrel is a piece rifle barrel made into a pistol 54 caliber 1" at the breech. Breeched with a 3/4 16 thread and a plug I made for hot rolled 4140 GB steel. There is a little black in the corner of the lands in a place or two. It probably shot about 50 rounds total for testing and fun.
It is perfectly sealed though there might be a "slight" leak at the vent. But the liner is installed on a shoulder and its pretty well sealed too.
Had I been able to make it 11/16 thread I could have had a perfect seal of the breech and the liner.
Sorry about the photo quality. Will see if I can re-do the bore scoping and get them so I can post on Photobucket.
Dan