Kinetic Destroyers are master-crafted Archaeotech Pistols, that are wielded by the Adeptus Custodes' Custodian Venatari. These gun-slinging flying golden eagle bois seem to carry these things around in bulk, which leads credence that these things aren't as rare as one might suspect an archeotech weapon would be, and instead they are weapons made exclusively to the Emprah's finest. To be accurate, archeotech means that is ancient and not reproducible, not necessarily "rare". For what we know, the golden boys might have stumbled into a Dark Age of Technology Winchester warehouse during the Great Crusade and loot 30000 of these and therefore have a lot laying around.
Anyways, Kinetic Destroyers obviously fires a physical round; it's in the fucking name. However, what type of weapon this thing is, is a source of Skub, it doesn't really help that it is an archeotech pistol anyway and we all know how esoteric these things are. At the very least, we know it is a giant revolver, like that of a Stub Revolver. But whether it is a Stubber or an Autogun is up anyone's alley. A third answer however, could state that these pistols are actually a form of Railgun or even a actual Gauss weapon (No, not the Necrons weapons which are anything but a Gauss gun).
On tabletop, the Kinetic Destroyers are the only pistols in Forgeworld Custodes guns. 18" Pistol 2 S6 AP-2 D2 can score an additional hit on unmodified roll of 6. MEQ/Primaris/TEQ killers with enough range to put you out of rapid fire range of most troops.
Seems to be the sister variant of the Kinetic Destroyers by the basis on having the same barrel as the Destroyer.
Like how the Guardian Bolter is mounted on the Guardian Spear, the Archaeotech Repeater seems to be a Kinetic Destroyer mounted on a Venatari Lance. In this case, the Archaeotech Repeater is a single barrelled Kinetic Destroyer that supposedly house less ammunition than the pistol version and lacks the revolver mechanism we see on the Destroyer. Overall, what has been said about the Kinetic Destroyer would already be stated here.
Rules wise, it is a 12" Assault 2 S7 AP-3 gun. Decent at softening up MEQs before going in to the meat blender with the lance. Usually a instant kill for GEQs so it is considered a waste for the Venatari.