Gauss Weapons also referred to as Special Propellant High Velocity Particle Delivery Weapons; SPHVPDW for short. Gauss weapons are meant to be an alternative to starting tech for laser weapons, with a different profile but roughly comparable and also available from the start. The main reasoning behind this decision is that both real life hand-held lasers and hand-held railguns suffer from the same problem: it is impossible to build a power supply compact enough to make the weapon practical. If we assume that this problem was overcome for the lasers (probably thanks to early research on alien tech), then we can make gauss weapons too.
The SPHVPDW project was started as a means to produce weapons comparable to alien plasmas, but without using so much - or preferably any - Elerium 115. While not Elerium independent yet from top to bottom, they're getting close, and the less 115 x-com uses in ground weapons the more they have for their Avengers, Firestorms, Fusion Balls and ship mounted weapons.
These weapons use non standard propellants. The bullets really make the weapon in this case, there’s no ignition pin, for example, or even a hammer. Instead, the guns fire small gyrojets that use a combination of electrothermal plasma ignition and special liquid propellant, five times more powerful than rocket fuel. Each bullet is caseless, the outer sections disintegrating when the metallic ring on the back end plasmates. Because the bullets are basically self propelled, except for the initial acceleration, there's relatively little recoil. Once it gets going, the bullet will supercavitate allowing it to even operate underwater.
Now, as for the rifles themselves, exotic alloy construction was still required to mitigate the intense pressure, bore heat fluxes and extreme surface temperatures the weapon is subjected to as the bullet accelerates and enters a full burn. There's minimal moving parts and it uses the ENL-5 lubrication you're familiar with. It should be good for all weather conditions, and if you take them into space, and there should be no problem with temp extremes or vacuum. It can handle four hundred degree thermal shock, which is a good deal more than you'll see in space or on Mars.
The one really unusual thing to note, is the anti-proton accelerator built into the base of the weapon. The real punch comes from a tiny stream of antimatter injected into the bullet casing before it fires. On impact, most of the round will disintegrate and ignite.
Each weapon has a series of internal diagnostic features to evaluate whether both the bullet and the rifle are 'safe to fire.' If there's something obstructing or interfering with the anti-proton stream, it won't inject, though there will always be a certain amount of anti-matter in the accelerator reservoir once it goes hot or active. There's a manual ejection port in case of a serious jam, but unless its something like flakes of alien alloy, any round firing will annihilate any matter stuck in the barrel. Barring that, the casing can snap open to allow replacement in the field.
Both the pistol and rifle are a bit on the heavy side compared to most battle and assault rifles, a little less than a re-chambered 6.7mm MSG-90 sniper rifle. There's also practical size and weight limitations on ammunition capacity per clip, since the individual rounds are larger and heavier than normal. The 12.7mm rifle round is about 400 grain, for example. The US models have twelve shots per clip for the pistol, eighteen for the rifles. As for the weight, it'll be light as a feather for someone in MARS armor.
These aren't a replacement for plasma carbines and heavies, except for the plasma pistol. These were made to fill a niche for rapid fire, 115-conserving, support arms. The Heavy Laser is still a beast when it comes to sniping and the plasmas are still heaviest hitters.
However Laser rifles don't have the refire rate for suppressive duties or room clearing, and really neither do plasmas. Tests proved that an enhanced sidearm needs high refire rather than the accuracy we get from the laser pistol or the raw power of the plasma. These next generation projectile rifles will supplant our existing stocks of laser rifles.
The NLP - Mk2E 'Next Gen Sidearm' is chambered for 10mm, specifically the XCOM Custom Automatic Multi-purpose projectile (MPP), classified as an armor piercing incendiary explosive. It has superior stopping power and penetration compared to the laser pistol and better fire rate than the plasma pistol. The plasma pistol, for example, fires a particle stream at around eleven kilometers per second. There's no way to match that with a projectile, unfortunately, but the specs for the Gauss aren't too bad.
Its speed actually increases with distance instead of decreasing, at least as long as it keeps burning and turning, so its more like a little rocket than a conventional bullet. The wolfram superalloy kinetic-kill core tip is designed to mushroom and shatter on impact, delivering massive amounts of kinetic energy to a target and causing catastrophic hydrostatic shock as the round over-penetrates at multiple angles. In terms of speed on impact, the rifle's 12.7mm will typically be reaching two point five kps at the 50 meter mark... the Gauss Heavy Cannon's 24mm round peaks out at around 4 kps, though, giving it a real punch. MPP rounds have the silver tip, SLAP have red. The ones with the blue band are tracer rounds.
The rest of the round is the gallium arsenide disintegration pocket. That's the section that contains and ignites the injected antimatter. The initial impact force is about the same as you'd get from a old school light fifty then on top of that you add in the explosive effect. Still not as hard hitting as a pure plasma, but pretty close.
The caseless block itself is very stable, water-proof, sand-proof, near soldier-proof. It will only ignite under precise conditions, so don't worry about it cooking off due to heat or anything like that. The barrel is technically self cleaning after every round fired, but tests have shown you should still manually clean it every two or three hundred shots.