1. go into crouch stance
2. open the inventory screen, do not close it now
3. drop all weapons except the handgun
4. lower the handgun. might have to press the button twice to get out of the "open inventory" animation
5. drop the handgun to the ground

You are now in an infinite putting-down-an-invisible-pistol animation loop.

If you now pick up the pistol again, you do a weird and long animation sequence which includes hand motions like you were holding a rocket launcher.