Grapeshot consists of masses of metal ball bearings fired at an enemy vessel using rail-guns. The bearings disperse widely in a very short period of time, allowing them to impact at numerous points across an opposing vessel's shields or hull. The bearings are far too numerous, and are employed at too close a range, for enemy defenses to have a chance to intercept them. Grapeshot is used primarily to weaken and bring down the shields of opposing warships, though it is capable of inflicting severe damage to unshielded vessels.
Grapeshot is usually fired twice in an engagement: once at extreme range, and again just before entering hell-lance range. This first shot is able to be dodged if proper deceptive tactics are employed (as John Geary does on multiple occasions). With the reload stated to take at least 5 seconds, this gives grapeshot an effective range of (at minimum) 0.5 of a light second = 150,000 km, although probably closer to 1 light second.