Mykeal is right, condition is critical. The bore light is a good suggestions but running a tight patch down the bore will tell you a lot. To me, light rust on the patch is not the end of the world, but the patch should go down the bore in a smooth and steady fashion. Any roughness can be felt and with experience you can feel even smoothed pitting. The bore lights that I have used give a good indication of the bore condition at the muzzle (which is usually the same for the entire bbl). That said I have had experience with several cases where there was significant corrosion at the breach and the muzzle was pretty clean. I believe this was due to the use of the older Italian made corrosive percussion caps.