"I don't see how a wad that doesn't completely seal the bore is of any value."

That's kinda what I was thinking, but then again, if a PRB sealed the bore why does adding a wad sometimes help?

I like your idea for fixing the size, though I'm uncertain how I'd know when I was there (.520"). I have a set of borrowed calipers, but I seem to have trouble reading the inside of my cylinder chambers accurately.