What do they hold? Ammo?
“They” are magazines, and they hold cartridges. Often, they are called “rounds,” but never “bullets.” Bullets are the small things at the tip that go faster than all get-out. Clips are special hardware used to hold rifle cartridges together, until loaded into a rifle. There is a version for pistol rounds, but one would never confuse them with “magazines.”
Ammo is good. Ammunition is better. Eh. To-MAY-toe, To-MAH-toe.
Just get with the jargon. :^)