From the introduction:
Sometimes an investigator just has “absolutely, positively got to kill every motherfucker in the room.” Used properly, many guns are capable of achieving this, but there are some designs that offer the extra punch that an investigator sometimes requires. Others are not really more powerful than more ordinary guns, but exude the kind of style that helps getting the job done.
The guns featured in this supplement are ridiculously oversized handguns, seriously cut-down assault rifles, shotguns with bulging revolver drums, and of course miniguns ‒ the very name a contrast in itself. All could be seen as answers to questions that no-one asked. Except ‒ except that for an Investigator of the Mythos, those questions might actually arise, and those answers may well save somebody's life ‒ if
not all of Mankind's!
From publisher blurb:
Overkill includes:
S&W Model 500 Magnum revolver
DSA Mini SA58 OSW assault rifle
Armsel Striker shotgun
TsNIITochMash SR-2 Veresk submachinegun
GE Minigun (M134) machinegun
Overkill works with Sixtystone Press’s Investigator Weapons 2: Modern, and any D100-based RPG like Delta Green.