Only happens if you switch hotbar selection directly after the gun is finished reloading, the bullets are counted server-side (meaning they're real bullets, that are loaded into the gun for real, and can be really fired) and it doesn't consume any bullet items to do this, meaning the only real limit is the 32 bit integer limit.

Easily replicated and easier to exploit, but I told ye in DMs and it seems you didn't remember sooo... onto the public issue tracker it goes!

Final two videos, first one showcasing regular RPG behavior (it still fires relatively quickly but yknow, not THIS fast. Plus it consumes ammo.)
Second one, showing how easily its done as well as the fact that it does NOT consume ammo

another video to hopefully drive in the effectiveness and severity of this bug

Fixed in 22.4.0.