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.