I'm guessing your using come custom PP expressions on your instanced geometry? If so please could you post them so we can help ya.

A simple thing to do would be to add a runtime expression that tells whatever your using to drive the rotation to stop when it collides with the floor.

You may have some odd results as the bullets are only instanced so it won't be them that are hitting the floor but the particles, so there may be penetration between the bullets and the ground which can't really be helped with instanced geometry.

As Rage mentioned a little more info would be handy.

Chris