Why do people refer to ammunition as bullets?

Just to clarify something you wrote, the 4 components to a cartridge are the the projectile, casing, powder and primer. A cartridge is a complete round. People use the term bullets for ammo for any number of reasons, mostly because it's the common use term for any loaded ammunition.

It kinda bugs me too.

Because the projectile (bullet) was for many centuries placed into the weapon which launched it solely by itself. It has only been a recent development that the bullet was part of something else which was loaded into the weapon as a whole in order to launch it. People simply continued calling the entire thing a bullet.

I shoot bullets, not the cartridge or powder, or the primer. So I just refer to it as a bullet and I'm sure it all dates back to the muzzle loading aria

I think it's because the bullet is the working element of the whole, kind of like we call tires "wheels" and shocks "springs"

Calibre is a measure of a projectile's diameter. It can be measured in mm (metric) or inches (imperial). To convert mm to inch, divide it by 25.4. To convert in to mm, multiply by 25.4. Most imperial named cartridges have a metric name as well e.g. .30 Carbine is also called 7.62x33, .303 Brit is also called 7.7x57R (R for Rimmed)

Ammunition, often referred to as ammo, is a generic term meaning (the assembly of) a projectile and its propellant.

I call it a round.... force of habbit from being in the Army.... What is weird is people call a magazine a clip..... you use the word clip in the military and you will do a million push ups...LOL... :)

The misnomer had become so by constant use.Even gun wise people do this.

I call it ammo or rounds