On my Walker, I took the nipples off, was replacing them with after market SliXShot stainless steel nipples anyway. I had some .454" soft lead round balls, so I loaded one into an empty chamber, it fit well, but did not shave the small lead ring off. I only pressed the ball in so it was just below cylinder face. Used a hard wood dowel to push the ball back out. The .454" ball had swagged down to cylinder chamber size, but had one small spot that had not. I tried a .457" ball, both Hornady's BTW, and it gave me the slight lead ring I wanted to see for a good seal. I was pleased to find all my chambers were of equal diameters. I also pushed a soft lead ball through the barrel, to check for tight spots or restrictions in the bore. Uberti recommends .457" on the Walker as well. I think .454" is a good starting point for your ASM '51 Navy. As long as you get a however slight lead ring cut when loading, your golden! If not, bump up to .457"
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