My experience is the bores run close to the caliber, i.e. the 50 caliber barrel was close to .500. Used .490 and .495 balls with .017 pillow ticking patches. On the 45 caliber barrels I used .440 and .445 balls.
Had a bench rifle (with a false muzzle) that had a .580 barrel with a .004 choke the last 4 inches. Used a .578 ball with teflon coated pillow ticking. The powder charge was 240 grains of FF. I had to make the .578 ball mold from a .575 by lapping it out. But that is another story all unto itself.
............P58 2 Band Parker-Hale 58 cal. Lands are .580" from muzzle to breech. Grooves at the muzzle are .590" (.005") and at the breech are (supposed to be) .013" deep for .616". I've not had the breech plug out to see for myself.
.502-.505 in the grooves, measured with a caliper, not slugged lead. I use a .012-.013 patch with a .490 round ball. It's tight enough to impress the cloth weave into the ball before the shot (I've had to pull enough balls to know this!)
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