I was astonished at the response for the first installment for this series, and so decided that instead of just a couple of survey pieces, reader interest justifies a bit longer treatment, getting more into the weeds. Thus, I have come up with a bit more structured series.
Unlike rifled firearms like pistols and rifles, shotguns are smoothbore weapons. That is, they have no rifling to impart spin to stabilize the projectile, in the case of shotguns usually multiple projectiles. Shotguns are a relatively recent development in their current form, but the concept of firing more than one projectile at a time goes back a very long time.
Modern shotguns are designed to fire a relatively large diameter cartridge as opposed to rifles and pistols. In addition, a different system is used to state the diameter of the cartridge in most cases.