I’ve seen a girl at my school say an ar-15 is a 15 caliber

That's sad but true.

An AR but smol bulets

Do you have the slightest idea how weak .15 cal would be? I've seen a .17, those are tiny.

Yes...I think everyone on this stream knows that.

the ar-15 is a rifle that was made by eugene stoner (just the name lol) and it there were products that were manufactured by ArmaLite hence the AR. the military adopted it as the m16a1. lots of other variants were produced: the m16a2, m16a3, m16a4, the m4a1 carbine, the m4, all based on the same design. There are other ar15 platform/based rifles being made by many companies, for example, the Honey Badger (a PDW thats integrally suppressed thats based off of the m4a1 carbine)

typical chamberings: 5.56 NATO, .223 Remington, .300 blackout (a stealth round that is mainly paired with a suppressor), .50 beowulf (kind of like a nitro express round), and in some rare cases 7.62 (trust me idk who would do that)