I agree with @JTam - looks like a fine homage to the Aliens Colonial Marines, by way of "heroic" 40k proportions.
I'm not sure that's meant to be a rail on top of the handgun, but a barrel shroud of some sort? That doesn't look like a slide at all - I don't think we can fairly think of it as a conventional semi-auto pistol powered by action/reaction physics: maybe a rail-gun, gas/air gun, or some other exotic technology, such as a Gyrojet gun?

Illustration: Gyrojet pistol - it doesn't fire bullets, but 12mm rockets; note the odd barrel shroud. A 22nd or 23rd century Gyrojet pistol, with under-barrel laser, flashlight, and/or stun gun, perhaps? Sure, why not?
For what it's worth, there were Gyrojet rifles/carbines as well:

Gyrojet carbine and rifle: The "microjet" projectiles leave the barrel at low velocity and temperature, and accelerate at about 30 feet from the weapon to a high speed - the barrel doesn't need to be very heavy, the projecties are relatively light-weight, and the entire weapon is very compact, light, and low-recoil. I'm not so sure the troopers' carbines aren't based on something like gyrojet technology, perhsps the grenade launchers as well....
I assume the "full battle rifle" ammo/mags are actually for the grenade launcher (analogous to the 30mm (iirc) grenades from the Aliens movie), but an alternative ammo to either "full battle rifle" ammo or Gyrojet "microjets" might instead be Smart Bullets, which might be expected to be a little larger than conventional intermediate assault rifle ammo, to allow for guidance system and fins; Smart Bullets are much more accurate over longer distances than conventional ammo (and much more accurate over longer distances than they are shorter ones), and depending on the AI of the guidance system, long-range optics might not even be necessary or even practical, since the bullet's AI would take over and home in on the target regardless of the shooter's accuracy. Besides, optics, it seems, are built into the troopers' helmets, no?
Gyrojet and smart bullet technology - or some combination of the two, or some futuristic electromagnetic weapon like a rail gun - could conceivably be easily fired from some of those awkward poses, too ;)
For whatever it's worth, I actually own one of the original 1960s Colt AR-15s with the flat sides, triangular handguards, slender barrel, and integrated "carry handle", and I can attest to the fact that only a kiddie-sized hand is getting into that "handle" :) The more modern detachable "carry handles" have even less room, to allow for the hardware to attach to a rail. The "handles" DO seem to do a great job of mounting and protecting integrated iron sights, though, and I used to own a '60s era M-203 grenade launcher sight that clamped onto the handle, as a collectible and novelty, and it seemed to be fine for that purpose. I'm not sure if it was actually designed to be a carry handle, but I kind of expect it wasn't - there are much easier and more comfortable ways to carry the rifle, starting with the pistol grip, and including the sling, forward handguard close to the magazine, and the narrow part of the butt-stock behind the handle. An elaborate sight-mounting platform seems far more likely, especially since later "flat top" versions of the rifle do away with the handle, for mounting rails for a wider variety of optics.
I assume that raised mounts for optics would be what the "carry handles" on futuristic weapons would be for, too.
Regarding the grapefruit-sized grenade, that's not very far off in size from the "tactical oxygen nukes" used by the movie version of the Starship Troopers, who are very similar in concept to the Colonial Marines (and an idea so over-the-top for an infantry weapon, it would look right at home in 40K!) These nuclear grenades also have a rocket propulsion and use a smart-bullet styleguidance technology, if I remember correctly (been a few years since I saw the movie....)