Dum-dum is a slang expression for several types of modified pistol
ammunition. A
Hollow point bullet is sometimes reffered to as such. Also normal jacketed ammo that have had notches carved across the top are also called by this term. The effect of the latter is for the jacket to fragment upon impact into four chunks, along the cross indentation. This creates a
shotgun type wound, with multiple exit points, and greater blood loss and trauma. However, the downside to altering a bullet in this fashion makes it less
aerodynamic, and thus less accurate at longer ranges.