Rail Guns. I know they somewhat exist IRL but they aren't a feasible long gun. I have such a Philip J. Fry type of love for them though.

I am working on a 'realistic' sci/fi mech setting. Mechs are mostly used for uban combat and still need infantry support so they use those goofy spring shoes that have probably killed a few people by now to stay a head and clear any anti mech weapons by parkouring around the city. The legs retract when they need to go inside a building.

Besides that though most of my weapons are pretty standard as I tend to focus on production history and why certain designs show up in certain parts of the solar system.

Like I have one mech that is sort of the AK or MIG of the setting. It was built to be cheap and so simple that you can train someone to use it (well enough) in six months where other mechs take three years of training. Its also built with the intention of having easily replicable parts so its easy to take a part and smuggle. It was built near the end of the Lunar-Earth war to quickly replace Lunar colonies heavy loses. It didn't save them, but at the end of the war they had more mechs then any other solar nation combined which have been sold off to rebel groups all around the solar system. So in the end the Lunar colonies fucked everyone over including themselves.

Another mech was designed by belter colonies who needed a standardized but easily customizable design to meet all the varied challenges they face in terms of industry. Theyn also though it was a good idea to use the same chassis for their weapons and so there are hundreds of 'rogue' colonies springing up they are so easy to get ahold of and modify to the point that they don't even need to be used just for urban combat.

You just made me realise my world didn't have any weapons at all in its worldbuilding! Maybe people would hit each other with sticks or something if they're angry, lol.

I mentioned the Gauss rifles on a comment, but I wanna talk about my atom-gothic setting too though it's not quite sci-fi.

During the forever war a lot of relics and items of magical origin have been seized for use on the front. Most people at this point don't have the piety/devotion/innocence/practice to use these things as they were intended, so creative engineers have found more unconventional uses for them.

An example of this is the hilt of Næġling, said to have been slapped by Beowulf in his battle with the dragon. When impacted by anything made of electrum it amplifies that force exponentially. Built into the stock of a lee-enfield it's now the personal weapon of Winston Churchill III on the German front. With the Germans and Australians raising the dead, it let's him fashion bullets from crosses salvaged from the churches on the front.

I suppose it depends on what you consider "interesting" - I tend to find relatively "conventional" sci-fi stuff like coilguns or advanced missiles, so long as they're fairly well designed! But with that in mind, let me discuss a few relatively unique ones:

• C+ Artillery is... exactly what it sounds like. Actually predating FTL travel by a fair bit (humans... go figure), C+ artillery hurls a massive slug through a sequence of faster-than-light spacefolds, like skipping a stone off a pond. The results are utterly catastrophic: Not only does the projectile bypass armor and interceptors as it "skips", but the target is subject to the unspeakable gravitational shear forces at the event horizon of the final fold... and that's before the enormous slug at .99C slams into it. They're used for cracking fortress-stations and dreadnoughts, and thankfully have never been fired against a planet.

• The typical infantry armor can be outfitted with the SPG-22, a 60mm, box-fed semi-automatic coilgun mortar. This system is mounted pointing vertically on the back, and is fed from a 4-round box magazine. It's most notable for the lack of need for emplacement: If necessary, the operator can kneel, the armor braces itself in place, and the mortar fires as necessary. Typically the user is actually aiming the system using an onboard firing computer, while an assistant operator keeps the weapon fed. In this way, rapid firepower can be pushed down to a platoon level.

• The AGS-202 'Easifa' ('Storm') is a monstrous cluster munition: Weighing in at over 3,000kg, it fragments into several individually-guided petals, which in turn try to arrange themselves for maximum coverage of an area... before they in turn disperse a mix of pyrophoric incendiaries, high-explosive fragmentation, and guided armor-penetrating bomblets by the hundreds. They're meant to erase entire defensive lines, although if you can disperse one over a base, the results are equally terrifying.