Most of the pistol bullet diameters vs caliber designation has to do with the original bullets were healed bullets making the bullet diameter the diameter of the OD of the case. When mfg went to traditional bullets, bullet diameter became the diameter of the ID of the case but they never changed the caliber designation. I've found this common with most old rimfire 'garden rifles' in 32 cal. I have to be careful buying balls for my .44 BP revolver. If I buy .429 balls they drop right into the cylinder.