I've considered the anvil theory as a possible culprit for the firing pin nose pitting problem myself but never came to any hard conclusion's one way or another. All I can conclusively say is that once I polished out the pitting and switched over to W-209's the problem went away.
I've now loaded a round worth of 20ga ammo with the same loading using W-209's rather than the Cheddite's and will get out to the range sometime this week to see how it goes with them in the same gun.
As far as the myth about interchangeable sizing between Cheddite and Winchester 209's I haven't found this to be an issue. I measured both of them with my calipers and found no difference in physical O.D sizes between the two. I did find that federal 209A's were slightly larger though.
The mystery to me is still the inconsistency in having piercing problems with the Cheddite's. Why only some of them and why only on some guns?
