It’s always a balance in games between granularity and ease of use. When it comes to weapons you need to balance the desire for different weapons to have advantages and disadvantages with the desire not to have everything become too burdensome.
I want to emphasis the skill of the character over the worth of the weapon, so bonuses will primarily come from character skill and development. A lot will come down to roleplaying, the description of the weapon and how it works.
A silent weapon, for example, may have no game mechanic effect but when used it’s not going to make any noise and so isn’t going to alert any guards (provided you manage to take someone out in one shot).
Standard weaponry is unlikely to stray beyond the -1 to +1 range of bonuses.
A friend of mine, Tim, has long argued that damage ranges on weapons should include a ‘1’, allowing for a full range from glancing blow to deadly hit and that’s been weighing on my mind while I’ve been restructuring my notes for MotSP and I think there’s merit to it. Weapons are, then, likely to have damage ranges from 1d4 to 1d12.
There are exceptions to the above of course. Exotech (pronounced Ex-Oh-Tek) are the relics and leftovers of past, advanced civilisations. They’re the ‘magic items’ of this universe and so they can break all the normal conventions.
Ammo I’m not so sure about. It can be a pain in the arse to keep track of so either ignoring it or abstracting it are options. How do you feel about keeping track of ammunition? Do you like to track it down to the last bullet or would something like an ‘ammo save’ after each combat suit you better? An automatic weapon would have a low ammo save, while more precise, single shot weapons would have higher ammo saves (roll under not to spend a ‘mag’).
I’ll include how weapon saves would work in the weapon lines below:
Example Weapon Line:
Burning Hand-Beamer, Accuracy +1, Damage 1d8 (heat), Range: Short, Ammo Save: 10
Burning Hand-Pulser, Accuracy +0, Damage 1d6 (heat), Range: Short, Ammo Save: 15
Postmortem Studios 
Re: ammo – well, some people seem to derive enjoyment, in some oldschool games, of making very detailed inventory lists, being particularily enthusiastic about weapon consummables (wand charges, ranged weapon ammo, etc).
Personally, I don’t like to waste much time about those kind of things, so I just give players ammo in the way of poker chips, where a chip acts as an equivalent of ammo enough for a short to medium fight. If the battle extends, then you have to give me additional chips. I think its a nice compromise.
Chips of different colors may represent different kinds of ammo.
I personally really like the idea of using the ammo save vs. Trying to track each and every round.
You could use natural roll of 1 (or a larger range for smaller ammo items) to reflect running out of ammo.
I liked Ammo Saves in games up until the player steps fresh out of the airlock, fires his gun once and runs out of ammo. It was certainly a game stopper moment where we just laughed at the rules.
In a super sci-fi setting, I lean more towards ignoring ammo altogether. I just assume everyone is using weapons that recharge or carry enough ammo. If people really want the crunch of ammo counting they can come up with their own numbers.
My concept of ammo saves would be ‘at the end of the fight’ rather than after each shot.
The same issue at least potentially applies, you can fire off one (or very few) shots in a fight and still end up failing your ammo save. At least ”by-the-book”, as I’d say the obvious solution would be to have the GM not call for an ammo save in such situations. There is also the matter of abstraction vs. player perception, as most players seem to be fixated on the fact that they only rolled one ”attack”, even when that represents their characters being involved in 60-seconds (or whatever) of intense gunplay…
I like your version of ammo saves quite a bit, but I think adding that aspect of running out of ammo when you roll a natural 1 on your to hit die also makes more of an impact on gameplay.
I’d suggest using both.
Perhaps expand ”running out of ammo” to ”problems with the weapon (or gear in general)”? This neatly avoids the problem of ”Wait? Why would my character only bring one round?” and could allow a greater variety in weapon characteristics (perhaps a letter-code to denote which problems are liable to happen with a given weapon? ”AAJOUU” would be a weapon that may run out of Ammo, Jam, Overheat, or suffer issued because it’s Unwieldy; I stuck to 6 codes so that it would be easy to randomize if you wanted, or else the GM just picks what is appropriate / dramatic. Oh, if you do this, be sure to include Indiscriminate, so you can blow stuff up you didn’t mean to…
I definitely support minimizing mechanics though, and particularly boni. Is damage type really going to make a difference, beyond as a special effect? I might even suggest that the bonus for a given weapon be less generally-defined. Some weapons are better in close-quarters combat, some for mid-ranged firefights, some for stealth assassinations, and some for long-range sniper duels… Perhaps instead a bonus in the appropriate situation, and/or a penalty in an inappropriate one?
I like the idea of an ammo save – it works given the handwavy futre-tech of the setting and easily represents e-clips, flechette mags or nano-transformable smar-munition packs with equal ease. That said I feel like pushing the ammo save to the end of combat exclusively robs firefights of some potential kickass action and drama. I feel like there should be a way to force an ammo save in combat in exhcnage for some immediate benefit – something to represent the protagonist kicking in the door an playing rip-a-clip with his pulse rifle. . .
If you want to add more granularity to the weapon stats maybe give each weapon a Rate of Fire – you can shoot (make attacks) up to the RoF each turn no problem, you can shoot up to double the RoF (x2 attacks/round) at the cost of making an ammo save; up to three times with a ammo save at a penalty and so on.
If the goal is to keep it spit-simple the hero can opt for an ammo save to score double damage (more bullets = more pain), get a bonus to accuracy (more bullets = more likely to hit) or to split the damage amongst a number of creatures equal to the total damage dice thrown (hosing down the room with lead). Most of the time PC’s won’t worry about ammo until after the fight’s done but if they’re trying to be awesome and put the big bad down hard they can rip through ammo and risk running dry. Turns a largely passive resouce mechanic (do I have to buy more bullets when we get back to homeworld) into something active (do I double tap this clown with style and risk having to reload while his goons draw a bead or play it safe and suffer him breathing another round while I plink away at his armor?).
You could also have “ammo hits”, where failing an “ammo save” checks off a box on the weapon, and they run out of ammo when (or right after) the last box is checked. Autofire weapons might have only two boxes, while a semi-auto rifle might have four or five.
Also, if you’re going to use an “ammo save” as the primary mechanic, at least have stats for ammo capacity/ROF, so someone could run a “X number of bullets; make every shot count” scenario.