As long as it's balanced and comparable to the other options that are available (bows, energy guns) and isn't complex for the sake of complexity or different for the sake of being different or "realistic" (PF gun rules), I'll be happy.
If you wanted to make guns balanced against bows, you could, for example, have free action reloading and a gage rating for each gun, and if your Strength is too low you deal less damage or have a penalty to hit (say, a cumulative -2 penalty on attack rolls until you spend a move action to steady your stance). I'd advocate either free action reloading (possibly requiring the Rapid Reload feat if the weapons are more powerful than normal) or ridiculously large clips (50+) so you don't have to worry about breaking full attacks due to rounding errors with how many bullets are still loaded.
For automatic weapons, you can either do Rapid Shot style (ie: more attacks, lower attack bonus) or d20 Modern's spray and pray style (area attack that uses a Reflex save instead of an attack roll). There are probably other options if you want them.
Since these are psionic items, rather than explosive-based, the "ratta-tatta-tat" sound is probably the auditory display of the effect, rather than the actual sound of the explosion that launches the bullet. So you can have a little fun there, with some guns that sound like real guns, some that sound like cartoon sound effects, etc.. Similarly, the muzzle flash is the visual display, not an actual explosion. With no explosion, blanks could actually be perfectly safe, if you want them to exist. Silencers are just display suppressors, like a Concentration check (which proficient users may even be able to suppress on their own).
