Hellgun
The Hellgun is an advanced variant of the IG lasgun. While the lasgun fires at a fairly rapid semi-automatic rate with a full auto setting, the hellgun is capable of firing extremely rapidly at full auto nonstop, allowing the user to bring the Empra's light faster and deadlier than the standard-issue Flashlight. Besides the fact that it needs a backpack-mounted battery to use, it requires extensive maintenance and is only issued to elite Imperial guard troops like the Kasrkin. The back-pack mounted battery gives a hellgun or hellpistol an effectively infinite amount of shots, which is very handy when you're facing orks, traitor guardsmen/heretics/cultists, and Tyranids. It's almost on par with a bolter, but it is way easier on the quartermaster due to the aforementioned limitless number of shots. For some fucktarded reason they are getting replaced by hotshot lasguns which are essentially just regular lasguns with supercharged shots. Ehhh, Dark Heresy has rules for both weapons. The newly released Cadian Gate Edition of the Imperial Munitorum Manual refers only to a Hellgun/Hellpistol. It describes them as more powerful than the standard las-fire and having excellent armor piercing capabilities. Full auto capability is also retained. It might be assumed that a hotshot lasgun merely uses the individual "magazine" hotshot power cells, while the hellgun is specifically designed to use the backpack-style "death-machine" capacitor. That or once again Games Workshop has failed to effectively manage its intellectual properties. Most likely, since the lore consistently has Stormtroopers wielding hotshot lasguns and Kasrkin as using hellguns and the art and models of both are different with the hellgun still looking like a hellgun and the hotshot lasgun looking like a bunch of cylinders getting smaller towards the barrel like a reverse telescope.
Probably the latter. As evidence, please consult page 8 of Codex: Militarum Tempestus, which describes the Ryza pattern Hotshot Lasgun. It is described as using "a hyper-yield power array worn as a backpack rig." The codex entry actually mentions it NOT using the maglike power packs in service amongst the Imperial Guard.
In light of that bit of ambiguity, the only thing that can be said definitively about the hellgun/hot-shot lasgun is that it is outperforms the standard lasgun in armor-piercing and damage at the cost of increased weight and scarcity. Otherwise its the stub/auto conundrum, again.
Update: The recently re-released Made-To-Order Kasrkin models describe their guns as Hot-Shot lasguns. It's an official name change. Lexicanum notes that the two terms are inter-changeable and the backpack/magazine power-supply is dependent on the mark. They otherwise are still select fire, armor-piercing and (in the fluff at least) deals more terminal damage than the standard lasgun.
Creed has two pistol versions of them on his belt. actually they are only hot-shot laspistols
What the heavy stubber is to autoguns and what the stormbolter is to bolters, the hellgun is to lasguns.
Hellpistol
The Hellgun's baby cousin. Often confused with the Hotshot Laspistol since their functions are roughly the same. Unlike the larger Hellgun, which is normally restricted to special forces units, Hot-Shot Laspistols are more common among high-ranking Imperial officials, including Imperial Guard officers or agents of the Inquisition.
There are two known types of Hellpistols.
Cadian-pattern Hellpistol: Associated with the fortress world of Cadia, this pattern of hellpistol has been adopted in places as far away as the Calixis Sector. Like other hellweapons the pistol draws energy from a 10 kg backpack powerpack, though it can also hook up to a larger 15 kg powerpack for more shots. Cadian-pattern hellpistols also incorporate an integral Targeter to assist the user while still allowing for a second sight to be attached.
Lucius-pattern Hellpistol: This Hellpistol is produced on the Lucius Forge World and is a common sidearm for many Imperial officials. It can connect to a standard 10 kg backpack powerpack or a larger 15 kg powerpack for additional shots.