Heavy Pistols are 3P, with the exception of the Ruger Super Warhawk, which is described as "shiny and chrome and makes holes the size of a troll’s fist straight out of the box", and the Ares Viper Slivergun, which uses "metal slivers that count as flechette ammunition and are bought as such, but they aren’t interchangeable with any other flechette weapon". Both of these weapons are +1 DV over the other Heavy Pistols.
This is similar to the Remington Roomsweeper being the only weapon rated at 5P, whereas all other shotguns are 4P.
I believe this has been discussed in several other threads, including the change blindness thread, as flechette ammo does -1 DV unlike the Slivergun (and potentially the Roomsweeper, depending on your reading). I don't think there is conclusive evidence either way, but based on the Slivergun I lean towards the Roomsweeper having +1 DV as Flechette was +DV and +AV in past editions.