How about a 20mm Vulcan -plenty of pain there.
That it is. But it's big heavy beast with a fearsome recoil. Although there was once a towed version, I think the only things that mount them now are aircraft, at least F-16 or larger helicopter size, and armored vehicles (a version of the M-113). I was thinking of a lighter replacement for the M-2, Maybe even one with a hand crank like the original Gatlings, or one powered by propellant gases somehow. But even failing those and electrically or hydrolically powered, something to mount on tanks, self-propelled artillery, wheeled armored vehicles and even the HUMMV and that V-bottomed armored car, where an M-2 is used today. The Vulcan is too big, for some of those applications, and might tip over the lighter ones if fired to the side. Maybe a 3 barrelled version in .50 BMG.