The most logical option for a future humanity would be railguns or coilguns, even today electrically powered weaponry is a viable (but still sub-par) choice. It seems to me that gunpowder weaponry is as good as it can get, and almost all upgrades or new innovations to modern weaponry are based around electronic additions or advanced manufacturing techniques.
With our current rate of technological expansion, electricity storage and handling will likely be good enough to surpass modern gunpowder weaponry by the time humanity goes interstellar, and since these guns just run on energy, the production and maintenance of such weapons would be a simple endeavour.
There are two types available, railguns or coilguns. Railguns are very simplistic, as you just feed electricity through the projectile, an on/off switch is all you really need. Railguns are prone to wear however, particularly the point of contact between bullet and rail. Replacement components may be needed if firing at high power and high rates. Also, the rails will try to force each other apart with each shot, requiring a strong design.
Coilguns are more complex, you need multiple coils to activate as the projectile passes through them, requiring more electronics and circuitry, and thus creating more points of failure. Greater speeds can be achieved however, and there may be less wear, as the projectile need not come in contact with the rest of the components, the coils can be wrapped around the barrel and stay separate. Finally, since each coil is separate and doesn't need to touch other components, they can be completely insulated from the outside world, but a railgun requires its high voltage rails to be open to the world, risk of electrocution, keep rain of the barrel!
Powered by electricity alone
Simple to manufacture, pure iron bullets would suffice for the ammo
Almost no moving parts, would only need them to load the chamber
Projectiles must be magnetic, so heavier elements or exotic rounds (incendiary/explosive) may be difficult to make
Extra points of failure. Electronics are more fragile than the machined mechanics of current weaponry, each component is likely a point of failure
Vulnerable to EMPs, such a tactic may be able to disable an entire army if not properly defended against. Coilguns in particular.