Brass

From GunCounter

A way of referring to the Case of a round of ammunition, but can be made out of steel or aluminum) is the part of a cartridge that ties everything else together: It holds the bullet and primer in proper alignment with the barrel and firing pin respectively, and protects the powder until ignition, and contains the expanding gases of the ignited powder and channels it to propel the bullet down the barrel.

The equivalent part of a shot gun shell is generally referred to as the Hull