I seem to recall someone asking what firearms in PF use to propel projectiles. Only the GMs can answer that question, of course, but I can think of four options. And I realize that this is not a high priority item -- the purpose of this thread is mainly to spark discussion.
A. Chemical explosives -- basically what we have now, though in the future there would likely be more energetic compounds available than what we have in the early 21st Century. Pros: It's simple, reliable tech that has had centuries of refinement. Cons: Current compounds are close to the theoretical limit of energy storage for this application. We can make more powerful explosives, but using them would also require that we make more robust firearms that can stand up to those bigger bangs.
B. Electrothermal propulsion -- this system uses an electrical current to vaporize a working fluid, which expands rapidly to the same effect as an explosive compound. Pros: The energy storage limit is higher than what can be achieved with chemical explosives, at least in theory. This means higher muzzle velocities and more energy for the bullet to transfer to the target (yes, twinks, that means more damage -- but only compared to current firearms). Cons: Electrothermal weapons would be more complex than purely mechanical ones (i.e. hydraulics to pump the working fluid), and thus require more maintenance. The system requires electricity (fairly high voltages, at that) and therefore requires a power supply of sufficient energy density that is small enough to fit inside the weapon. This begs the question of how much electrical storage systems have been miniaturized in-setting. Some possible working fluids are toxic and/or dangerously volatile.
C. Magnetic drivers -- "gauss guns" use a magnetic field to accelerate a projectile to very high velocities. A railgun has two parallel conductive tracks along which the bullet runs, while a coilgun has a series of coils which the bullet never touches. Pros: These things are ridiculously powerful. The theoretical limit for muzzle velocities for this kind of weapon is about Mach 15 (compare to Mach 2 for modern firearms). They might also be recoilless, or at least have less recoil than you would expect from such a fast-moving projectile (the magnetic field is the action, the movement of the bullet is the reaction... at least, that's one theory). Cons: Gauss guns suffer from the same power storage issues that plague electrothermals... but on steroids. These things eat a lot of power, so they would either not get very many shots or man-portable electrical storage systems in PF are very, very good. The acceleration requires a barrel of whatever length the bullet must travel to get to muzzle velocity, so there would likely be no pistol versions. Magnets (powerful ones, anyway) are heavy, therefore so are gauss guns. A bullet leaves the barrel of a gauss gun with enough speed and friction to ionize the air -- the resulting muzzle flash emits bright visible light, an infrared flare, and a significant magnetic field at the same time. And that's before the sonic boom you get from a projectile traveling at hypersonic speed. There are no silencers or flash suppressors for these things. The combination of these drawbacks means that gauss guns are likely support weapons. With the power issue alone, it's reasonable to restrict these to vehicle-mounted weapons, crew-served ordnance, or artillery.
D. Some combination of the above -- Some of these technologies might only be available at the higher firearm classes. I can definitely see gauss guns being milspec only.
So... comments? Concerns? Overripe fruit?