Just a logical offering but think on this.

many eons ago when people were loading from powder horns and bullet pouches, there was a singular item not connected to anything else called a bullet.

Fix it to the top of a combi of cartridge with, as has been noted, shoulders and a neck....head is a logical name for it.

you can see why people call it a head, or a bullet, bullet head is perhaps for those that call rounds or cartridges bullets...and lets face it many of us do.

So what the does it matter?