The R-10 (1-2 decades pre-fall)
The SRDA's Terraforming and Colony-building Prospect is phase 2 of The Plan, started by the Theia, Tanu and the Aurea long-haul ships, which gave us access to the Ilus and New-terran systems. For mobility, industrial use and self-defense, they have heavy power-armor mechs, equipped to move cargo, carry out heavy ship repair and deal with the local fauna.
Originally built for military use, main arm for these mech's would've been the infamous R-10 Heavy Railgun. Shooting depleted uranium flechettes, it would be reminiscent of its terran predecessors, its magazine being ahead of the grip and the buttstock housed the electrical storage and switching system.
As it would have been operated by the mech driver, it would have power, control and cooling supplied directly from the suit, with the refractory super-materials used in the long-lived hall-effect engines being put to good use, with cryo-coolant being passed through channels inside the rails, like how composite resistive magnets (Florida Bitter designs) are cooled.
An iteration on this is having a secondary superconductive rail behind the main rail, which carries the current and negates the resistive heating constraints as the current only travels from the outer rail through the inner rail and to the projectile.
The K-10 (1 decade post-fall)
At the end of the first expedition into what was left of the lab, to cover an injured Kit and the space-plane trying to perform an extraction, he hauls up a discarded R-10 and fires it, injuring himself while disintegrating everything on the action-end of the rifle.
Do remember that he is a keidran. Even a gruff marine or burly mechanic guy would have trouble using one of these rifles without the mech-suit to boot. The average keidran (of which basically all are doggo-wolf breed) would usually have about 7" or so on the average human, and so it would be conceivable that a keidran marine could heft it, if it where self-contained.
But Jayden is a fox keidran, being a lot smaller, definitely had problems with a 150 pound (70ish Kg) gun. And after about 2 months of engineering and lots of sore shoulders later, created the K-10 Railgun Rifle.
The K-10 'Kitsunebi' Carbine is the first man-portable variant of the R-10, making it a rather unique all-purpose light assault weapon, with high anti-material performance. Identical to the R-10, it uses (albeit much smaller) flechettes, which come in enormous magazines, which unlike the R-10, are loaded behind the grip and trigger, coming in small cartridges of over 100 rounds.
In place of the magazine, it has the port for a standardized crystal-lattice power cell, which only store enough power for around 20-30 shots. Above the grip is the computer-aided holographic scope, which connects to the HUD of a helmet, ship computer and/or space-suit, when used.
With it's otherwise high muzzle velocity of around 2 to 4 kilometers per second and near-perfect accuracy, it doubles as a makeshift anti-material sniper rifle.
Unsurprisingly, the phase-gate ties in, as the technology needed to quickly source and sink the mass-energy equivalent of an entire mountain range is ironically very similar to that needed for a railgun. Like the R-10, the buttstock houses the electrical storage and switching system, mostly copied from the damaged phase-gate.
Unfortunately, the last and most problematic issue of this weapon (from a writers as well as an engineers perspective) is the rails and how to cool them. Since active cooling is impossible without a mech suit and all the options it gives you, the engineers (Jayden) had to get creative.
Instead of having the flechette contact the rails directly, they would have a small pad on either contact that would be vaporized as it enters the barrel. This tiny layer of plasma would bridge the gap and let current pass without wearing the rails out. (At least not any faster than conventional rifles.)
In Japanese folklore, kitsunebi (Lit. fox fires) are orbs or streaks of blue fire as methane from decomposition burns, usually appearing over marshes and wetlands. Similar to will-o'-the-wisp, they are attributed to Kitsune, magical anthropomorphic foxes of folk stories. Related to the companionship and taming of foxes in ancient Japan, they are (in some stories) portrayed as guardians and faithful lovers.
A result of the solution would be a trail of glowing blue plasma with each shot, which would look quite spectacular at night and in vacuum. As you can imagine, that is where the name came from, and I am putting up a 100 rep bounty for whoever got the reference before the explanation.
How well thought out is this weapon? Is there any glaring issues I might have with it? I don't have a single question in particular, more of whether or not any of it is flawed or flat out backwards.