Anyone using firearms in space will be presented with one major problem: heat. Guns on Earth can rely on the convection of the atmosphere to transfer heat from their weapon. In space, a barrel can cool itself only through thermal radiation, which is not fast enough for sustained fire. Heat buildup can weaken or deform the barrel, is dangerous to the user, and makes users obvious targets in infrared. Present-day designs use meta-materials to handle or radiate large amounts of heat, or even convert it to electricity, but older designs can still be found in use throughout the system.
The Hydra line of late 21st century firearms solved the heat problem by using multiple barrels. Their primary product uses typical intermediate rifle rounds but is bulkier due to its three barrels and Gatling gun mechanism. Despite its three barrels overall rate of fire is kept low. The heat increase of each barrel is thus a third what it would otherwise be. Sustained use will still build up dangerous amounts of heat, however.
The Drunkard line of rifles and machine-guns uses liquid cooling, surrounding its single barrel with a water-tight drum which can be flooded. The liquid, typically water for convenience, although the system is designed to accept any fluid, rapidly absorbing heat from the barrel. The reservoir of coolant also acts as a reservoir of heat, greatly increasing the amount of heat that the gun can generate before problems occur.
All of these guns are considered kinetic firearms. The GM may rule that they cannot accept smart ammo or take full advantage of smartgun systems if they wish to emphasize their obsolete designs.