Thank you Grenadier1758 , Even with your explanation I'm still scratching my head. How did it get started "way back when"? Its become a industry standard for 45 balls to measure smaller than 44s , how did it come about? It almost seems like simple math was left out of the equation or something???
First off, the size of the guns barrel caliber is the same size as the
bore of the gun. That is, a .44 caliber cap and ball pistols
bore size is .440. A .45 caliber rifles
bore is .450.
Because the
bore size in a .44 caliber pistol is rifled the rifling groove size is larger, usually somewhere around .450.
Because the ball in a cap & ball pistol must engage the rifling, it needs to be larger than the bore size and preferably slightly larger than the rifling groove size.
To allow that to happen, the chambers in a cap & ball revolver are about the same size as the rifling grooves. (Although they should be the same or slightly larger than the groove size, most Italian replicas make their chambers slightly smaller than the rifling groove size.)
In order to keep the ball in the chamber and to form a tight seal with it the ball needs to be slightly larger than the chamber. That will allow the excess metal to shear off of the ball as it is rammed into the chamber making it a very snug "line to line" fit.
These cap & ball balls are therefore usually around .451 to .457 diameter.
Getting back to the .45 caliber rifle and any of the true muzzleloading single shot pistols. Because the ball needs to be rammed all the way down the bore, it should be smaller than the bore.
Because the smaller ball won't engage the rifling grooves or seal them off on the way down or back out, a cloth patch is used to do this job.
This allows the smaller .440 or .445 diameter patched ball to tightly engage the rifling and seal it with the patch doing the job.