4X magnification shows you interior barrel fouling, nicks and damage up to 16" from either end. Just shine a light source in the opposite end and inspect any barrel or chamber .22 cal. and larger.
Exellent tool for checking the throats for carbron fouling and wear. It does have a learning curve but once you get it right, the thing works great. Just don't expect to use very deep in the barrel. You have to be able to hold some angle on it to see the lands and grooves good. I almost returned it when I first tried it, thought it was a useless, Then I bought a 20" fiber optics adapter for my mini-MagLite. It lets me get the light just right so I can see everything I need with great clarity and detail. Too bad they don't have a 45 degree adapter for it. Then I would have given it five stars.
Good inexpensive scope. Good magnification and clarity, works well with carbine sized barrels or smaller. Too short for rifles with 20" or larger barrels
This little tool is very fiddley to use. However, with perseverence, it does provide some additional information about the condition of a barrel. It has limited depth of field and a decided learning curve to recognise what you are seeing in the dim light available when looking down the bore from the muzzle. Trying to look at the breech end of a barrel is even more difficult but it is possible to obtain some limited information about rifeling and throat condition. A few minor changes could result in a significant increase in usability. Inparticular, if the view was at 45 or 75 degrees to the bore it would be easier to interpert what you are seeing. To sum things up this is an inexpensive tool, it is not a replacement for a $700 bore scope; you get what you pay for.