Here I will be looking at Slegehammer's own unique designs only, not the slightly-rehashed ARs and SMGs like the SAC3, which is clearly just a futuristic take on the Vector, or the AMR9 which is obviously just an AR-15 chambered in 9mm. This is for Sledgehammer's fictional guns only.
The Integrated Munitions Rifle (IMR) is a very unique design from Sledgehammer, although how exactly it works is less than clear. From what I can piece together, it seems that the 3D printer unit (called the "Formcaster 3D") built into the stock inserts newly-printed munitions straight into the barrel, one after the other via a revolving mechanism. Basically, the user racks part of the barrel into the Formcaster and each round is stacked into the barrel.
The obvious question is "how does it fire"? Well, confusingly, Sledgehammer claimed that the gunpowder is pre-loaded, but also that the firing mechanism relies on the coilgun principle. The latter makes sense - multiple rounds stacked into the barrel could fire one-after-the-other via a series of electromagnets wrapped around the barrel - but this would mean that the weapon would not require gunpowder whatsoever. Coilguns don't use gunpowder, they fire solid projectiles, which is what the IMR appears to do. So why is there muzzle flash, recoil, and a loud gunshot?
Real-life equivelant? Concept art reveals that the MTAR was used as a base for the IMR, but the finished product resembles the AUG more than anything. The similarities are purely cosmetic, though; there is no real-life rifle that prints its own ammunition.
Plausibility? If it were a true coilgun, then it would be possible - in theory. But it certainly wouldn't be in military service by 2050, if ever. It's the kind of thing that some US Army scientists are going to blow a few million dollars on in the future, with only a malfunctioning prototype to show for it.
The Atlas 45 appears to be a .45-caliber handgun manufactured by or for the Atlas Corporation, although it appears to have had a lot of commercial success too: it is used by the United States Marine Corps and the KVA. In terms of design, it looks like a miniature version of the Vector, but operates like a regular pistol.
Real-life equivalent? None, but the action is basically that of a Mauser C96 in the body of a Vector.
Plausibility? Not very likely. It is front-loaded, which is a poor design choice for a handgun; front-loaded pistols were pretty much phased out after WWI because pistols with magazines placed in the pistol grips allowed for longer barrel length and thus better accuracy. In real life, the Atlas 45 would be inaccurate, needlessly large, and very heavy. The only advantage I can see is that the magwell can be used as a grip and thus recoil could be dampened. The Atlas 45 is perfectly plausible in terms of design, but there's no reason for it to exist in real life, except maybe as a novelty commercial pistol. Definitely not military-grade stuff.
The Atlas 20mm, like the Atlas 45, appears to be manufactured by/for the Atlas Corporation. Strangely, however, the Atlas 20mm is never really seen or used in the campaign whatsoever. As it's a massive, shoulder-mounted 20mm cannon, it's probably intended as an anti-material rifle for taking out light armor.
Real-life equivalent? It's just a generic mish-mash of various shoulder-mounted AMRs, although the overall appearance is quite similar to the Cuban Mambi, bar the magazine feed.
Plausibility? Almost none. There was a time when massive-caliber anti-vehicle rifles were popular, but that time has long since passed. These days, 20mm rounds would struggle to penetrate the armor of a battle tank, so it would be next to useless in the 2050s. It's interesting to note that the Atlas 20mm has tally marks on the side (presumably for each kill), but among the tallies are two jets etched onto it, implying the user has shot down two jets. An infantry 20mm rifle as an anti-aircraft weapon? Against high-speed jets? Unlikely.
The EM1 is a directed-energy laser gun that fires a constant beam that burns any surface it comes in contact with. In the campaign, it's used by the Atlas Corporation and by the Sentinel Task Force. How exactly it works is not made clear, but it behaves like a generic sci-fi laser weapon - there is no ammo, just a battery that can overheat.
Real-life equivalent? None. Although laser technology has been experimented with for military application, it's ultimately too complex and too costly to be condensed into the form of a small arm.
Plausibility? I think I can safely assume that no such weapon will be in service, let alone exist, 50 years into the future. If the in-game performance is anything to go off, then the EM1 is a pointless weapon; it's massive, presumably very heavy, very weak (at least in the campaign mode), incapable of penetrating even thin surfaces and also quite noisy. I can't see any advantage to such a weapon existing.
This never appears in the campaign mode, only in the multiplayer and survival modes. It is essentially a very large single-shot plasma rifle of sorts. Like the EM1, how exactly it works is never made clear.
Real-life equivalent? The technology itself does not exist, but Sledgehammer seem to have taken design cues from the Denel PAW-20 grenade launcher, which has an overall similar shape and the exact same trigger set.
Plausibility? Like the EM1, I can't see any advantage to this design. It overheats very quickly, has an incredibly bright muzzle flash, isn't very powerful and has a lot of recoil. The beams of plasma that the EPM3 produces after each shot would also probably give away the firer's position. And, given that it's basically just a worse version of the already-poor MK14 in the game itself, I don't know why Sledgehammer bothered with this one. Futuristic tech for the sake of futuristic tech, perhaps?
The XMG, like the EPM3, doesn't appear in campaign. Understandably so - the XMG is incredibly inaccurate unless the player enters "lockdown" mode, in which they are rooted to the spot. That probably wouldn't work well with the flow of the fast-paced campaign missions. The XMG (marked as the XMG-1 in game) is a pair of miniguns that are fixed onto the user's arms, making them a kind of human turret. They are fed through massive drum magazines that are reloaded off-screen via unexplained means, and also curiously have aiming reticules despite the fact that the user can't aim them. XMG probably stands for something like "eXperimental Machine Gun", similarly to how "XM" stands for "eXperimental Model" in US Army nomenclature.
Real-life equivalent? The miniguns are not modeled after any specific weapon; they seem to just be generic Hollywood-style miniguns.
Plausibility? The XMG is not going to happen. There is little argument to be had here. What is up for debate is whether or not the XMG is actually intended to be dual-wielded or whether it's just a forced multiplayer trait, similar to the SAC3. The XMG has a holographic sight, a cocking handle and a detachable drum magazine, indicating that when the weapon was designed by the boffins at Sledgehammer's design department, their original design was intended to be used as a standalone weapon.
I thought I'd bundle these together, since they're essentially the same weapon in two different forms. Both the RW1 and the MORS are single-shot railguns. According to the campaign mode, the MORS is in service with most major military powers by the 2050s, as it appears to be the most common sniper rifle. The RW1, on the other hand, only appears once. The RW1 is break-action, which is a system usually reserved for large-caliber competition or hunting pistols, certainly not military weapons.
Real-life equivalents? None whatsoever, although the RW1 is clearly inspired by competition pistols. The MORS, on the other hand, is only in the game for the sake of there being a 1-hit kill bolt-action sniper rifle to appease trickshotter-types.
Plausibility? More so than most of the other weapons listed, but still very unlikely, certainly by the 2050s. Massive, stationary railguns are being experimented with currently but the railgun system is generally not suited to small arms because it would consume far too much energy per shot and would be too costly to develop with not nearly enough benefits to justify that cost. Coilguns, although in their infancy, are far more suited to the small arms role.
Back to the MORS and RW1 though - correct me if I'm wrong (and I may well be), but I thought that railguns did not require gunpowder, and thus their ammunition would not need casings? The RW1 seems to eject a casing upon firing which is wrong on more levels than one (it completely defeats the point of it being break-action), and the MORS relies on a bolt-action system which is bizarre because the point of the bolt-action system is to strike the primer of a cartridge that the ammunition of a railgun would presumably not even have, so it could just be magazine-fed or semi-automatic. Am I missing something here?
The Bal-27 (what "Bal" is supposed to mean, I have no idea) seems to be a P90 and an F2000 merged together. The body of the weapon resembles the F2000, but the internal mechanism is that of a P90, with a top-loaded flat magazine. It seems to be the standard-issue rifle of the Atlas Corporation in the 2050s.
Real-life equivalent? As mentioned before, this is a mish-mash of a P90 and an F2000, but is only really similar to the F2000 in terms of cosmetics, so the P90 is a closer equivalent.
Plausibility? Some. While obviously Sledgehammer probably opted for the P90 feed system purely because it looks cool, it is actually pretty nifty for a few reasons. One of the main problems that people have with bullpups is that the magazine is hard to reach, which is somewhat averted with the P90 system - while still offering the same compact dimensions of a bullpup rifle. Secondly, and I don't know if the Bal-27 actually does this in-game, the P90's system ejects spent cartridges through the bottom of the butt, meaning it can be used by left- or right-handed shooters. As a close-quarters special ops weapon, it doesn't seem too ridiculous.
Honestly, I'm not 100% sure what the S-12 is supposed to be. The fact that Sledgehammer named it the "S-12", of all names, causes confusion because there already was a gun called the S12 in Black Ops II. Treyarch's S12 was just a Saiga shotgun, but Sledgehammer's is some sort of bullpup fully-automatic shotgun. Is it supposed to be a bullpup Saiga in a modern shell? I don't know.
Real-life equivalent? The "Kushnapup" shotgun, which is a Saiga in a bullpup shell, is as close as you're going to get to a real-life version of the S-12. The Kushnapup, like the S-12, can accept drum magazines.
Plausibility? Hard to say without actually knowing what it is, but it's possible... maybe. For balancing reasons, the S-12 is pretty weak and has abysmal range, but that doesn't excuse the rather pathetic 8-round drum magazine. Seriously, why bother modelling a drum if you're going to give it 8 rounds?
In-game nonsense aside, there's no reason the S-12 couldn't exist, but there's also no real reason why it should, as a military weapon at least. These days the military really only reserve shotguns for breaching doors. The S-12 could be a pretty effective room sweeper, like the AA-12, but the chances of hitting an unarmed civilian would be too high. I can only really see the S-12 existing as a commercial shotgun for shooters.