protoolman said:
Im betting its just bullet lube. I shoot a lot of cast and thats just the way it is. nothing to worry about. I have not seen a GP100 with alignment issues but I guess anything is possible.
According to a gunsmith who specializes in accurizing black powder revolvers, ALL manufacturers have cylinder hole to barrel alignment issues and the only way not to have such issues is to place a jig into the frame, cock the hammer and drill into a blank cylinder. Cylinders are usually made independent of the frame and alignment issues are certain, particularly when the bit chatters. At least according to my gunsmith.
If you take an EMPTY revolver, cock the hammer and look down the barrel into bright sunlight, you can actually see misalignment on Italian black powder revolvers, usually on 1 or 2 chambers on a good gun. Also note that the Italian guns use a RIFLE twist barrel, rather than a 1:16 twist, appropriate for a revolver. This is the reason that barrels are replaced on accurized revolvers.
Bill Ruger knew all that and designed the Old Army accordingly.