Page "Artillery" Paragraph 26

Propelling charges for tube artillery can be provided in one of two ways: either as cartridge bags or in metal cartridge cases.

Generally anti-aircraft artillery and smaller caliber ( up to 6 " or 152. 4 mm ) guns use metal cartridge cases that include the round and propellant, similar to a modern rifle cartridge.

This simplifies loading and is necessary for very high rates of fire.

Bagged propellant allows the amount of powder to be raised or lowered depending on the range to the target.

it also makes handling of larger shells easier.

Each requires a totally different type of breech to the other.

A metal case holds an integral primer to initiate the propellant and provides the gas seal to prevent the gases leaking out of the breech, this is called obturation.

With bagged charges the breech itself provides obturation and holds the primer.

In either case the primer is usually percussion but electrical is also used and laser ignition is emerging.

Modern 155 mm guns have a primer magazine fitted to their breech.