In Halo series games, a headshot with a 'Human' sniper rifle, or S2 AM sniper rifle. The rifles caliber is 14.5mm. Thus, a 14.5mm brain surgeon, is a good sniper.
Sniper to opposing player: "Well i dont know if i can fix your level of stupid, but I would like to see the effects of some 14.5 mm brain surgery!" (THis statement is either following, or is followed by, several sniper rifle shots.