Apologies in advance, this is going to be a bit of a long read.
I am working through the process of fleshing out a science fiction world and want to establish the limits of technology fairly early on, one of these technologies is weaponry and I'm having a hard time deciding on the function and extent of man portable firepower, which is fairly important to the story.
Lets establish a few things humanity has already accomplished, to give you an understanding of the technical level achieved.
Several moons and planets have already been settled and terraformed, with terraforming equipment being able to improve habitability by up to 50% its original state.
FTL travel is possible, but limited to certain lanes that open and close depending on the orientation of solar systems as the universe rotates on its axis.
Ground vehicles have by in large abandoned wheels, instead they use repulsor tech to keep the vehicle suspended off the ground by about 20 inches. This technology is highly efficient, and stays on even when the vehicle is not running, however the vehicle is lowered to about two inches off the ground to conserve power. Given extended periods without use, the system would eventually run out of power and need to be jumped. These ground vehicles range from small single-seat craft to vehicles the size of a semi-truck, but are exceedingly cheap. One the size of a family sedan would only cost about as much as a consumer motorbike.
Aircraft have risen substantially in popularity, and use a combination of modern flight principles, and the repulsor tech mentioned above that makes the amount of training and skill required to fly these craft substantially lower, and the safety has also been massively improved. In-Atmosphere shuttles are owned by most upper middle class families, and are heavily used in planetary public transit and business. Specialized lanes have been implemented to keep air travel organized.
Economical commercial spaceflight and public transit has been achieved, and costs about twice your average plane ticket to jump between planets, and about five times your average plane ticket for FTL travel to other solar systems. Costs for private citizens to own personal space capable craft is similar to modern day luxury yacht pricing.
Effective particle based shielding to protect against radiation on all space capable craft has been achieved, and this shielding is also moderately effective at stopping microscopic debris moving at very high speeds.
Combat grade shielding has only been around for about for about 15 years, and are much to expensive for any private citizen to afford. Despite this, large corporations and military's have seen large adoption of these shields. These shields are restricted to spacecraft, large aircraft, and large armored groundcraft. They require continuous power and lots of it. They function by suspending a mesh of high energy particles around the craft using strong magnetic fields. These mesh's react similarly to a Non-Newtonian fluid, and are capable of stopping almost all but the largest purely kinetic projectiles. However each impact scatters particles from the shield in large quantities, and the shield will have to move particles from around the shield to fill in the missing portions, causing it to lose density. When a shield becomes so thin that it is no longer effective, the generator will shut down.
The first energy based arms are coming onto the market to counter this, and exists as a hybrid kinetic/energy explosive shell. This technology works on very similar principals to shields themselves. Large shells fired from mounted turrets or cannons contain their own small energy field generators in the rear of the projectile. These generators are activated by the rapid rotation applied when fired from a rifled barrel, and sheath the projectile in a swath of high energy particles. These energy sheaths react much more strongly with combat shields, and can penetrate much quicker than standard projectiles. Usually shells that penetrate will still have their energy sheaths ripped off by the shields.
These shells exist at their largest in 800mm, and smallest at 30mm. There are single shot recoilless rifles, similar to a Carl Gustav, fielded by infantry that fire these shells for anti-tank purposes, and there are single shot 30mm rifles that are capable of penetrating the shields of combat shuttles and are designed to damage engine components/kill the crew. However at this time all other man portable firearms are still purely kinetic.
This is where you lovely folk come in. With this general level of technology, how would you envision standard arms like rifles, handguns, and scatterguns? My only wish is the the firearm itself should be as close to purely mechanical as possible. Modern weapons get all the energy they need to function by burning gunpowder, meaning that you never have to plug in your rifle to recharge, and if you have a dud round you simply chamber a new cartridge and your ready to rock and roll. I can't personally see people abandoning the reliability of this system. So whatever propellant, or energy you devise must contain all of the power to deliver the projection and work the action.
The actual story being told in this universe will take place on one of the most distant colonies from the Sol System, so reliability and ability to manufacture more ammunition and arms without support from Earth is crucial.
You can create your own elements/minerals/compounds that have any characteristics you want. If you want to use electronic tech such as railguns, make a compelling case that the electronics in the rifle would be highly durable and easy to repair while still keeping all of the energy in the ammunition cartridge itself. I hope you guys can come up with some ideas, because I'm having a hard time myself.