Over the years there has been a lot of discussion about revolver top strap flame cutting in smokeless magnum loads. Usually shooters are advised to avoid combination of light bullets and max. loads of slow burning powder. I've seen far less discussion about black powder loads and I have thought this to be because of its lower pressure not capable cause such a bead blasting effect.
Yesterday I was again at range with my still fairly new Uberti Walker. Its round count is now 84. I've only fired "mild target loads" of 30 grains (27 grains actually if weighed by scale) of Swiss FFG powder and round balls. However, to get the ball closer the chamber mouth I have topped the powder charge with another 30 grains of semolina. This load is very accurate and has never dropped the rammer. However, after 54 rounds I noticed some cutting at the arbor and now after 84 it's becoming more like a groove.
Like all cutting, I suppose this to become self-limiting at some point but still it is surprising to see the cut developing so fast. I can only speculate that it's the semolina that does the blasting job.