I broke down and bought one of those cheap ($14) endoscopes off Ebay for looking inside my guns. You get what you pay for with these. But for $14 I didn't expect a whole lot. But it does provide a previously unavailable view and is kind of fun to play with.
Here are a few shots. L-R top row: are a Spiller & Burr revolver bore, a view inside the receiver of an 1873 Winchester, and the breech of a Springfield model 1855 musket (look close you can see the flash hole to the left of the breech plug.
Bottom row L-R: the breech of my Remington rolling block .58 cal rifle, the same rifle with a .58 Roberts dummy round in the chamber.
Its a pretty neat little tool!