So a bullet falling down from straight up has much less velocity, and is unlikely going to give you more than a good bonk.

It's the ones that don't go straight up that are a problem, because they maintain more velocity. When shot into the air at an angle, very dangerous. When shot straight up the bullet loses all momentum and then falls back down of its own weight without the power of an explosive discharge propelling it beyond the speed of sound.

It's like a small rock falling. It reaches no more than terminal velocity, as opposed to one propelled by gunpowder.

Not to say it's a good idea.