I took a chance on a Clearview Engineering Borescope for a gun show tool. Simple, cheap and surprisingly gives excellent clarity. I still have normal near vision and the instructions suggest trying the scope with and without glasses if needing readers.
For me, the focal point is only about 2" from the end of the 8" tube, but that would still allow detailed examination of the chamber/cone/about 7" of proximal barrel and 10" at the muzzle. Obviously it moves freely in a 12g barrel and you can almost get a 3-D sense of the pits
https://4570products.info/Clearview-Brass-Bore-Scope-16.htm 
