The combustion of gun powder is slower than sound speed, it doesn’t mean the bullet cannot be though.

loose gunpowder* but yes, gunpowder actually burns very slowly. in fact, we have designed gunpowder to burn slower than old blackpowder from the musket days. it’s the containment and pressure that makes it work in a gun. but it’s also why it’s not uncommon to actually shoot some amount of unburned powder from many guns. a thing that was difficult to know or prove before high speed cameras.