Firearms are one of the consistent inventions across the majorty of sentient life. Every sentient race that has engaged in the killing other sentient life has, eventually, invented things that could be considered "firearms".
A firearm is defined as any weapon that is readily carried and used by an individual that fires ranged projectiles. In the RFI universe, firearms fall into one of several categories. Each has their own strength and weaknesses.
Conventional Projectile Weapons (CPWs) are, simply put, weapons that accellerate a projectile of some form (generally metal) at high velocities. CPWs are simple, reliable, predictable, and generally safe to operate. They are still the bread and butter of planetary defense forces, and other civilian oriented uses.
These old-school "slug throwers" are still some of the most prolific, reliable, and cheap weapons around. They might not have the armor penetration of other weapon types, but they aren't designed for that. On the other hand, if you're looking to make something made from flesh not alive anymore, they'll do the job just as well as anything else.
Railguns are considered by some the ultimate form of CPWs. They have most of the benefits of Directed Energy Weapons, as well as their cartridge brotheren. A railgun works by accellerating a metal projectile to ridiculous speeds in a very short amount of time. This acceleration is often achieved by machnetic means, however gravitic railguns are equally possible; they just require significantly more energy.
Railguns have a very interesting property that their damage is proportional to the amount of power put into the shot. The upper bound of this is generally the point when the atmosphere is turned into a plasma, or the destructive force is such that the one firing the weapon is caught in the resulting explosion.
Note: Take even the most basic, low power railgun and modify it with access to external power, and you can generally get a single shot of immense damage potential. Generally this comes at the cost of destroying to weapon, but it's been used by many a desperate individual.
Railguns represent the vast majority of light to medium duty firearms. They dominate the civilian market, and have made their way into paramilitary use as well. They are one of the weapons prefered for shipboard action as they can have their power dialed back enough to avoid puncturing the hull with a stray shot, while remaining fairly lethal.
These are traditional railguns, but their ammunition seeks to solve the 'power and ammunition' problem. The look like cartridge bullets, however instead of the body being filled with a checmical accelerant, it's an individual, per-shot powercell. While this limits the weapon to just the maximum output of the cartridge (though an external powersource can always be used to increase available power) it greatly reduces the awkwardness of maintaining both power and ammunition.
In most CRGs, the spent cartridges are collected back in the same clip or magazine as the fresh ammunition. This allows for them to be reused rather easily. Often, military versions will have battlefield ammunition reloading stations that can, in a few minutes, replenish the power for the cartridges, and reload ready-made projectiles.
Directed Energy Weapons (DEWs) are firearms that do not fire a projectile in a traditional sense. Instead, they use their own power systems to cause damage in other ways.
All DEWs are considered complex weapons to manufacture. They require intricate control systems and power management. While it's possible to make "dumb fire" CPW, all DEWs are "smart" weapons, integrating targeting, power control and other features. Since they require such complex control systems already, adding other smart features was simple.
DEWs are rarely used by civilians, with Puled Energy Projectile (PEP) weapons being the primary exception. Simple, non-lethal versions are sold for 'personal defense'.
Plasma weapons are devastatingly destructive. They bring the one-two-three punch of thermal and kinetic damage followed by electromagnetic radiation that can damage or disable electronics. They are incredibly power hungry, however. For the same amount of power, a railgun will generally get twice as many shots.
Pulsed Energy Projectile (PEP) is a type of weapon that fires an infrared lazer at an abaltive taget, generating a plasma projectile. Originally concieved of as a non-lethal weapon, PEPs can be scaled up to devistating levels.
Because PEPs are driven by a laster ablating a special material, PEPs are much more energy efficient than their larger, more destructive brotheren. A single weapon can have a non-lethal setting, as well as more destructive settings. They can be scaled down to be appropriate for ship-board action, as well as used to deliver impressive single-shot damage, with a PEP handgun able to deliver one or two shots at the same damage output as much larger heavy weapons.
It should be noted that since PEPs require ablation, they do need to be properly maintained, with their ablative plate replaced on a semi-regular basis.
Coaxial Plasma (CP) weapons generates toroids of plasma and fire them at hypervelocities. These toroids are self-reinforcing, keeping the plasma coherent over a longer distance than other plasma weapons. CPs use massively more energy than other weapon types, as the plasma needs to be generated (either from atmosphere, or an internal fuel source) and then it needs to be accellerated at a significant fraction the speed of light. This means that the damage potential of CPs over energy required is logarithmic. Normally, that would be a negative, but CPs are so devistating, they're almost never operated in the upper range, and are actually reasonably efficient in the lower power bands.
Note: Since CPs require a fuel to ionize, most military grade CPs have reserve hydrogen canisters, or reserve atmosphere tanks where they collect air when not surrounded by vacuum, and store it for use, if needed.
The main problem with CPs is the simply fact that they are the most complex firearm type. Many consider them unreliable; this isn't stricktly true, they just have to be maintained well and poorly designed weapons might be more sensitive to environmental hazards that other weapons.
CPs are used heavily in military actions where ungodly amounts of power in a small package is required. However, CPs are less desirable for use onboard a ship, because while they technically can be scaled down to the same ranges (and effects) as PEPs, it add complexity, so most firearm manufacturers simply don't design for it.
Lasers have never proven very practical, in terms of firearms. They are very efficient on power usage, and they have amazing penitrative power, literally able to vaporize away the thickest of armor. They can even be fired in space! However, they have a fundamental weakness: They can be defeated by a classic smoke screen, or a simple reflective surface.
That isn't to say that you can hold up something shiny as a shield and be fine. Even if 99% of the laser is reflected back, that's still generally enough to melt or destroy your mirror after a few shots. But the issue becomes, whatever you managed to deflect is now going off in another random direction (or worse, coming back at the attacker).
Most races have experimented with laser weapons, but none currently field them as firearms because of these issues.