I recently bought a pretty nice borescope that allows me to see straight down the rifling and at 90 degrees to see the walls. The resolution is pretty good as well.
So I have started pulling out my older C&R rifles just to experiment and take a look at the bores on them. I've seen a very broad range of bore conditions - many are completely independent of the external condition of the firearm as well. I know it was common to have older rifles use corrosive ammo and/or corrosive primers so that if they weren't cleaned properly to neutralize the chemicals, the bores would corrode, rust and otherwise deteriorate.
To my surprise, I found that my all-original 1905 Winchester Model 1894 SRC in 30 Winchester has a superb bore that's close to a mirror finish (very shiny) with no pitting and strong rifling.
On the other hand, to my dismay, an Arisaka Type 99 that I bought here a year or two ago that is in great condition externally with intact mum, AA wing sights, etc - has an abysmal-looking bore. By far the worst of all my C&R's. I've never shot it and simply took the seller's word on the bore condition.
I've also found that my previous method of checking bores is absolutely a POOR indicator of condition. I had one of those bore lights that you can bend up into the chamber to look in from the muzzle and see the lit-up bore. In almost every case, that resulted in a bore that looked acceptable and didn't ever really show any rust, pitting, frosting or imperfections. At most, it would let you see how prevalent the rifling was and not much else. After using a borescope, it really shows how poor of a job those lights do.
Moving on, I tried to find a good online guide to describe bore conditions and terms that had associated pictures. Alas, I am unable to find such a guide. Perhaps my google-fu is weak, but there seems to be much disagreement in forums as to what the terms mean definitively... things like "frosted", "rust", "pitting", "corrosion" etc. I think, given my new borescope, I can easily determine what rust is. Pitting seems obvious to me: the small depressions that form in the metal over time. Corrosion seems to be a more generic term that can encompass all of it, or something else entirely. "Frosting" is probably the one I am most confused on. In some of my rifles, I can see that the bore is clean - no pitting, no rust... but it's not shiny - it's more of a dull metal finish that doesn't reflect much light, albeit still "clean" with good rifling. I assumed that was what "frosted" meant. However, most forums seem to define it as a synonym for a pitted barrel.
Anyone know of a good, definitive source for barrel conditions that include pictures?
Also, for fun, here's the inside of my Type 99.