For those who understood what I was trying to post, thanks, I was too lazy to look up my books and go into detail.
This stuff originated over 500 years ago, not just in the last two centuries as most people here are stating.
Bullet diameter and caliber (as well as bore & guages) came around in the 1300's and 1400's at the beginning of infantry firearms. Measurements could differ by each country, county, city or village. This didn't bode well for soldiers or armorers.
Weights were standardised from ancient Imperial measures.
Weapons were sold by what weight of shot they fired. It was easy to sell guns based on what how many balls-per-pound they fired. Musketballs were 8 - 16 to the pound, pistolballs were 12-24 to the pound, etc.
This is why there are what seem oddball calibers like .72 cal, .69cal, .68cal, .44cal, .36cal, .22cal. These were close enough for any smoothbore of that size to function.
This just carried down to modern times.
