The Mage Hand cantrip is described as follows:
Range close (25 ft. + 5 ft./2 levels)
Target one non-magical, unattended object weighing up to 5 lbs.
You point your finger at an object and can lift it and move it at will from a distance. As a move action, you can propel the object as far as 15 feet in any direction, though the spell ends if the distance between you and the object ever exceeds the spell's range.
It says you can move an object "as far as 15 feet in any direction", but it doesn't specify how quickly you can cause something to move. I note that the spell does't stipulate the movement takes place over any particular duration, even the duration of the spell. And that makes me wonder.
Q: What do you call a tiny pebble weighing a fraction of a pound, accelerated to sufficiently high velocity over the course of less than 15 feet?
A: A bullet.
What's stopping players from using this spell (an unlimited-use cantrip!) to turn their local environment into an ammo box and wreaking havoc on any encounters they may come across?