can you identify this rifle

[Battis]

As far as the endoscope - I bought one for basically the same reason, to see what was in the breech end of a barrel. Well, they don't work so well. You need to get a strong light source down the barrel and then if you do get an image, it's very grainy. Maybe an expensive one would work - bring the rifle to your next colonoscopy.
A friend of mine had a double barrel shotgun hanging in his garage, and for the heck of it I dropped a rod down one barrel and sure enough, it hit something hard. I worked at getting whatever it was out for quite awhile with different poking and scraping tools. I'm still not sure what I got out but it was old.

[Ballistics in Scotland]

Maybe an expensive one would work - bring the rifle to your next colonoscopy.

It's an ill wind...

[LUCKYDAWG13]

Attachment 200250 Attachment 200251 Attachment 200252 A few more picks I had to drill the top off of one of the screws to take the side plate off

[LUCKYDAWG13]

a few more picks Attachment 200254 Attachment 200255

[Ballistics in Scotland]

Well that nipple isn't worth trying to preserve. It is probably hardened though, which makes itwa difficult to drill, and especially hard to drill without the sort of heat or spark that might set off powder. I can think of one or two ways around that. If you clamp it in a vice you could build a little dam of modelling clay and drill through water. Or you could epoxy a drill into a long piece of metal tubing and drill through the bore obstruction instead. If you need a carbide drill for the nipple job, a cheap masonry drill, ground to sharp edges with a silicon carbide stone or diamond hone, is good enough for one or two jobs.

The inside of the lock doesn't look at all bad. As long soak in a special penetrating and easing oil, such as Kroil, will probably help in getting it apart. Holding the shank of that beheaded set crew in a powerful vice should let you get it out, and that is better than drilling if you need to measure the thread pitch. Don't give up too easily on finding a replacement. But if you can't, tapping the hole to the same or marginally larger diameter and different pitch should be all right. Those screws don't bear great stress.

If you warm the lock and paint it thickly with wax where you want to preserve the patina, you can use a rust remover such as naval jelly on the inside. Or actual paint if you want to immerse it in stronger acid, which might warm up. About 10% hydrochloric shouldn't hurt it.