The following article is from 2015...
The gray lump that’s labeled 1 is the bullet. Typically, these contain a lead core that’s enveloped in a harder metallic case. Bullets poke out from the top of a shell (2), which also houses gunpowder (3) or a similar substance.
Now, look at the shell’s bottom. Down there, you’ll find both a rim (4) and a primer (5). When the gun’s trigger is pulled, a firing pin strikes the primer, causing the powder to ignite. As it burns, it rapidly releases gasses that build up with enough force to launch the bullet out of the cartridge and through the gun barrel.