Normal shotgun rounds are filled with a number of pellets; the smaller the pellet, the more you can fit. Birdshot: small pellets in large number, Buckshot: larg

Normal shotgun rounds are filled with a number of pellets; the smaller the pellet, the more you can fit. Birdshot: small pellets in large number, Buckshot: large-ish pellets in small number. A slug is a massive (18.5 mm diameter for 12 gauge) single projectile used in a shotgun shell.