Generally, shell is used to describe explosive projectiles/payloads or ammunition for shotguns.
In contrast, bullets are specifically projectiles that are sent through the barrel of a firearm when the propellant inside a round/cartridge is ignited. The casing (which contains the propellant and holds the bullet in place) is what is ejected after firing. The round/cartridge is the package as a whole.
Generally, shell is used to describe explosive projectiles/payloads or ammunition for shotguns.
In contrast, bullets are specifically projectiles that are sent through the barrel of a firearm when the propellant inside a round/cartridge is ignited. The casing (which contains the propellant and holds the bullet in place) is what is ejected after firing. The round/cartridge is the package as a whole.