The powder is contained within the case. The bullet is seated in the mouth of the case. Caliber and Gauge Ammunition is designated as caliber or gauge. A cartridge is designated in hundredths of an inch, thousandths of an inch or in millimeters. The caliber is measured across the bottom or base of the bullet.
Shotgun ammunition is designated in gauge.
One of those balls is said to be 12 gauge and would fit into a 12 gauge barrel. Therefore, the higher the gauge is, the smaller in diameter the barrel is. There is also a shotgun that is designated in thousandths of an inch and that is the .410 gauge shotgun. A shotgun shell consists of a plastic hull with a metallic bottom. In the bottom of the shell is a primer which is used to ignite the powder.