Hellgun
The Hellgun is an advanced high-powered variant of the IG lasgun. While the lasgun fires at a fairly rapid rate comparable to an autogun, the hellgun is more or less a man portable MG-42 that shoots goddamn lasers, 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 power supply to use, its power conduits and optics also burn out faster, requiring extensive maintenance, and because of this it is only issued to elite Imperial Guard units like the Kasrkin or elite Grenadier units. The back-pack mounted battery gives a hellgun or hellpistol continuous fire without reloading, which is very handy when you're facing massed Orks, traitor guardsmen/heretics/cultists, and Tyranids in high-intensity combat situations. It's almost on par with a bolter, but it is way easier on the quartermaster due to needing recharging instead of ammunition, although periodic maintenance does require spare parts. 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. 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. On the other hand, it's entirely possible that hellguns and hotshot lasguns are both equally high-penetration, high fire-rate weapons and are just different STC weapons each with their own pattern variants. Like how Stormbirds are all the same type of plane and all do the same thing, but there are many different patterns of Stormbird.
Nevertheless, this is dubious and contradictory as Kasrkin and many grenadier elements of other regiments (except Krieg, whose grenadiers use hot-shot lasguns) still use Hellguns whereas Stormtroopers (Tempestus Scions) use hot-shot lasguns instead. Since both are mentioned separately for different types of units, they are most likely not the same thing. 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. Newer art has hellguns looking like hellguns and hot-shot lasguns looking like lasguns with an extended, ventilated barrel attached to the tip and varies between hot-shot power packs and backpack-mounted with cables.
They are obviously different weapons now, though whether or not there is a significant different in armor-penetration is unknown. Since Krieg lasguns can crater plasteel and Krieg grenadiers use hot-shot lasguns instead of hellguns, perhaps hellguns have similar penetration to normal Krieg lasguns and so would be largely pointless for Krieg grenadiers and so a higher penetration weapon was issued. On that note, if it is cheap enough to be issued to the most expendable soldiers in the galaxy in massive numbers, why the fuck isn't it standard issue to the Guard in general? Literally every other regiment is both smaller and less expendable than the Kriegers. Use hot-shot heatsinks and hot-shot lasgun power packs and hot-shot focusing lenses in a hellgun so the lower energy won't damage the lenses and the maintenance problem preventing general issue of hellguns will be solved. The need to reload instead of just continuing to blaze away is solved simply by the numbers of the Guard. In fact, the lower numbers of soldiers equipped with hellguns is probably why backpack capacitors are used instead of far cheaper power packs anyway so that continuous fire can make up for the lack of numbers.
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. Which is unsurprising, since the IG power packs are not hotshot powerpacks (except those used by the long-las). Hotshot powerpacks are their own type and used by some types of hotshot lasguns. Presumably any kind of lasgun can use them, though, but a hotshot lasgun would probably consume all of the charge from a normal powerpack with one or a few shots or maybe draw more than the powerpack can provide with who-knows-what results.
In light of that bit of ambiguity, the only thing that can be said definitively about the hellgun/hot-shot lasgun is that it 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 and RPGs at least) also deals more terminal damage than the standard lasgun. The fact that GW did this is a further indication of their FAIL whose attempt to fix the Hot-Shot/Hellgun logistical debate was to state that they are "Hurr Durr. One in the same!". As you can imagine, not only was this pointless, but it ended up creating unnecessary confusion especially for Imperial Guard players who took a few years break from 40k only to come back and realize that an entire weapon family has been retconned. There’s also that little detail that Dark Heresy has them as two separate weapons with their own rules. GW has since slightly retconned their retcon by stating that there are two models used by the Imperial Guard that are quite distinct in performance, and the Lucius-patterned “Heavy” model is the one used by Grenadiers and with the Hellgun nickname as opposed to the Ryza-patterned.
And so, the difference between hotshot and hellgun remains ambiguous. Hotshot lasguns consistently being described as using heavy shots to penetrate armor whereas hellguns always use rate-of-fire plus above average power to penetrate armor. Basically opposite methods for different needs or purposes. Think building different weapon systems from the same original platform i.e. the AR-15 receiver (or Johnny Guardsman’s lasgun) being used as the basis for something like a higher caliber Designated Marksman Rifle (or hotshot) or a full automatic Light Support Weapon (or hellgun). If you accept this speculation as fact, then this means that the Hellgun should rightfully be seen as one in the same/a variant to the hot-shot volley gun instead of the base lasgun. Oh, and to further muddy the speculative water, hotshots are supposedly long and somewhat squat whereas hellguns are fat and stubby. So yes, very, very ambiguous. I guess we’ll never know.
Tl;dr. What the heavy stubber is to stubbers and what the stormbolter is to bolters, the hellgun is to lasguns.
As an aside, the Exarchs of the Swooping Hawks sometimes use the poetically-named Hawk's Talon, a weapon strikingly similar to the Imperial Hellgun in function, if not quite in form. As usual for the damn elves, it's better than ours in most respects (range, rate of fire, sheer punching power) but the old Hellgun still edges it out in performance against enemy armour.
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 quite common among high-ranking Imperial officials, including Imperial Guard officers or agents of the Inquisition.
Creed has two pistol versions of them on his belt.
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-kilo backpack powerpack, though it can also hook up to a larger 15-kilo 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-kilo backpack powerpack or a larger 15-kilo powerpack for additional shots.