Assume that modern day Earth discovers a way to create portals to another planet. Travel between the two planets is fast, safe, and easy.
The way this works is that there are fixed points on earth that corresponds to fixed points on this other planet (dubbed alterearth) where, if one builds a stargate-like device, one will be able to create a tunnel between the two worlds (this only work between our Earth and alterearth so no galaxy spanning human empire yet). Step into the portal on one side and you come out on the other planet.
This of course created a rapid expansion of humans to the new alterearth. Each country that has one of those fixed portal points has expanded around the matching point on the alterearth. Within 10 years after the initial discovery, the alterearth borders have been set with the 39 countries that had those portal points, covering 100% of the alterearth land mass.
Life on alterearth is a lot like life on earth, it has breathable air, land that allows for plants to grow, and the local wildlife is quite similar to Earth wildlife (not 100% the same and nothing is more intelligent then a primate).
There is only really one main difference between Earth and alterearth, the alterearth has a particle dubbed "ridiculousium" by scientists, which fitting to its name acts in really weird ways:
- ridiculousium is tiny, so tiny that in fact it's impossible for it to interact with pretty much every known material as it passes between every other material's molecules
- ridiculousium doesn't allow for fire to burn, it's not really clear how but it's impossible for fire to burn when near any ridiculousium particles
- same goes for explosives as it is basically a rapid ignition
- ridiculousium is super common and doesn't move; its molecules are everywhere on the planet and they seem to stay in a constant position in regards to the rest of the planet, like floating in that exact spot
- just to make it clear how much they don't move it's been calculated that every step a person makes he passes through billions^billions of ridiculousium particles
- Aside from the no fire/explosion thing ridiculousium doesn't seem to affect anything else
Day to day civilian life isn't really affecting by being in a world full of ridiculousium. The only real changes are that all power is produced via clean energy, cars are all electric, and all cooking is done on electric hot plates.
While day to day life isn't that bad off due to ridiculousium being around (one could argue it even helps as it takes away multiple major sources of air pollution), the new colonies there are all still part of their original countries and they all have armies on the alterearth to protect their colonies from other countries there. As the average army pretty much relies on explosives (which don't work on the alterearth) and firearms (which just got the "fire" taken out of them as it's impossible to avoid having a ridiculousium particle in the bullet casing and thus make it a dud) which requires a major retooling in the army side.
The question (at last)
Given this world, what kind weapon will an army supply its soldiers instead of the usual rifle? Remember that, other than the discovery of this portal transport, this is RL modern tech level and the army doesn't have years to wait to develop a solider carried rail\coil gun to use... this has to be something that can be fielded to the troops in a month or two at most.
Just to clear things up if there is not fire (as in flame) ridiculousium will not affect it in any way, it will not affect burning of calories or metabolism in anyway as there is no flame involved.