Apache 30mm M230 chain gun, lobs 30x113mm bullets down range at 805m/s, 625 rounds/min effectively up to 4500m.
M230 chain gun - Wikipedia, the free encyclopedia
Pretty much everything you would want to know details wise.
Snapped this photo earlier today.
Left to right:
12ga 2 3/4" shotgun shell (unexpended)
50 caliber (unexpended - fired from the OH-58)
20mm (expended - fired from the AH-1 Cobra)
30mm (expended - fired from the AH-64 Apache)