dumdum bullet
A bullet with deep user-made cuts in the tip. The cuts help the dumdum bullet fragment into chunks on impact, causing severe injuries and bleeding on a scale normal bullets would be incapable of. By  definition, dumdums are less accurate than regular ammo.

Dumdum bullets are illegal for use in warfare and may be illegal for civilian ownership in the United States.

Also see dum dum.
The cops say Jimmy got shot in the gut with a dum dum bullet... it took him like five minutes to die, man - he was bleeding that bad.