>I have a 36 Caliber Pietta that mics at 350 on the bore size<
Well, your problem comes from the way that you define caliber, at least in part.
When measuring barrels, the caliber of a weapon is measured as the bore diameter, which is the land-to-land diameter. One does not measure the projectile or the groove-to-groove diameter.
Case in point: land-to-land diameter is 0.360"--that is a 36 caliber bore. It should not be measured with a dial or vernier caliper because of induced error. And a decimal point should not be used in front of the number when the word caliber comes after the number. Caliber, in this case, means being expressed in hundredths of an inch, so the decimal is redundant.
Then figure grooves at about 0.0075" on each side or maybe more for a muzzleloader, which usually has deeper grooves than a smokeless weapon, and you end up with a groove-to-groove diameter of 0.375"--which corresponds to the ball diameter. However, with a capunball revolver, one needs a slightly oversized ball for a flash seal and friction to make the thing stay put in the cylinder, so 0.005" oversize makes it 0.380" or so.