Thanks, never really noticed it before, and at first thought it was a mill mark, but it's from gas escaping around the cylinder. The worst one is an old Pietta 1860 army that I'm refurbishing and was pretty much worn out. I don't know how many shots went through it, but the rifling is still good though the cylinder chambers were worn to where they would no longer shave a ring of lead from a 451 ball like they used to. The half moon grove was about 1/32" at the deepest, right in line with the bore. I would estimate no more than 3-4 thousand rounds through it, but that's just a guess.
I've got a relatively new 51 navy that has had some where on the order of 5-6 hundred rounds through it and the notch is only a couple thousands deep and about that in width, the only way I noticed it was because the notch is polished and the pin is case colored. I've got a 61 navy that has around 12-15 hundred rounds through it and it's about twice that depth, though only a little wider.
I should say that I've run a lot of ammo through them, at least 70-80 rounds a session, so they do get shot quite a bit.
I'll keep a watch and if it gets bad enough I'll do something, was thinking that since the area in front of the cylinder is rebated a piece of stainless heat treat foil wrapped around it would probably protect it for a couple hundred rounds if neccisary. I don't think it will be a problem for the foreseeable future, but have never heard anything about it before.
I suppose that if it takes on the order of say 20 thousand rounds to render it unsafe it'd be pretty cheap to simply replace the pin.
I only shoot black powder, until recently Goex, but have since switched to Graf's and Swiss.