Geno,
I agree, .001" in the choke is not too hard to get to with sandpaper or high volume shooting. The abrasiveness of powder residue and fricion of the wad would seem to be the wear mechanism IMO. I haven't noted any forcing cone erosion in my shotguns, but I don't have huge numbers of rounds thru any of them except one that has had the forcing cones lengthened after much of that shooting. Certainly, I, and maybe a bunch here have seen erosion in rifles and handguns. I had a .264 Win Mag that the throat and bore (for a few inches) looked like it was sandblasted after I had fired maybe 1000-2000 rounds thru it.