I purchased a "bore scope" from Harbor Freight Tools. The tip is 8.5 mm in diameter and permits me to look down 45 cal barrels with ease. As I move the scope and light into the bore, I see on the screen what is just beyond the lens on either side of the scope. They go for about $80 and I bought mine on sale for about $65.
There is one on the net which is identical except that the bore scope is 5.5 mm and can be used for 30 cal rifles. It also is more expensive at closer to $200.
It has changed the way I clean my traditional muzzleloaders. I followed instructions learned here and on other muzzleloader sites, using water with some dish soap and swabbing the barrel with patches until the patches came out clean. I even tried the infamous Moose Milk. When I looked at those "clean" barrels with the bore scope, I was shocked! There was crud throughout the groove corners of the rifling, and plenty of it. A simple bore light did not show the dirt in the grooves. So, now I still use the water and soap, but also a nylon bore brush that gets the gunk out of the rifling. I also wrap a patch around a bore brush and that works so much better than a patch alone. The bore scope confirmed it.
I know many of you have used the patch and water method successfully for decades and have no rust or other problems. However, now that I have seen for myself what is left in the rifling of my rifles, I just take the extra step. It simply makes me feel better when I don't see black particles in my grooves.
Ron