Originally Posted by JohnKSa
The change away from heeled bullets also helps explain the apparent disconnect between some caliber designations and the actual bullet diameters commonly associated with them. For example, the .38 special uses a .357" diameter bullet. The reason is that originally, .38 revolvers used heeled bullets, with a nominal bullet diameter identical to the OUTSIDE case diameter. Later, when heeled bullets fell out of common use, .38 revolvers transitioned to non-heeled bullets with a nominal bullet diameter identical to the INSIDE case diameter which was about .357". Same deal with .44 revolvers which typically use .429" bullets.
Another example illustrating the discrepancy is from the world of black powder revolvers. A .44 caliber BP revolver takes a lead ball that starts out around .452" to .454" in diameter, and shaves it as you press it into the cylinder. The conversions that are sold to allow .44 caliber BP revolvers to fire modern ammo use .45 Long Colt ammo.
The BP designation of .44 caliber was based on the bore, while the .45 Colt designation was based on the groove diameter.
Confused enough now?