The four components of a round of artillery ammunition. 

1. the fuse

2. the projectile 

3. the propellant

4. the primer

1. Fuses are the devices that trigger the artillery ammunition. 

2. The projectile is the munition or the "bullet" fired downrange.

3. The propellant propels the projectile to the target.

4. Upon being struck with a huge force, a primer reacts chemically to produce heat, which ignites the propellant and fires the projectile.