Well you said that you "assumed" the breech area would be .502" or .5015". Did you measure that Maxi-ball that was started from the breech? Was it actually smaller?
It could be roughness, fouling or as you say, a narrower bore. I'd try cleaning the living snot out of that area and see if it gets closer to the average of the rest of the barrel.
Perhaps I should do a similar test just to see if mine are that way.
