I wasn’t sure which forum this question would go in, so I flipped a coin and ended up here…
I need a suggestion for a slightly futuristic weapon that could be used for a character in an SF setting–about handgun-size, and not a laser gun nor a Star Trek phaser knockoff. I cannot for the life of me dredge any ideas out of my brain.
Let’s see, how about a pistol that fires razor-sharp disks? Perhaps something that fires an expanding capsule of sticky fibers to entangle the target? There’s the old reliable gas gun (kaff kaff!). A gun that fires concussion bullets, like mini concussion grenades? Hand-held flamethrower? Electrical weapon of some sort, similar to a taser?
Please note all but one of these ideas are lifted from various other sources (including perhaps a Star Trek novel or two).
How’s that for a start?
In Dan Simmons’ Hyperion/Endymion saga (which I highly recommend, BTW), there was a weapon called a flechette rifle. It fired thousands of needles at the target. Not a pretty sight, I’m sure.
The railgun is always nasty. Uses electromagnets to fire a projectile at nearly the speed of light.
A gun that fires a “smart” projectile that homes in on a specific target, i.e. a bullet with your name on it.
None of these are original, of course.
Then of course, there are pocket-sized railguns (you’ll need to assume semi-magic power-sources) firing everything from flechettes to glass beads.
Wire-guns are a variation of flechette guns, using spools to high-density wire, power fed and cut off in 1" or 2" segments, with the force of the cutting jaw sending the wire on it’s way (kinda like what happens when you snip the end off a piece of wire with side-cutters)
How about biological projectiles, maybe firing a fast-growing yeast that causes the victim to blow-up like a balloon, or maybe a gun that fires deadly insects?
Maybe a weapon that launches poison capsules, or narcotics?
Simul-post!
How about a Magnum-style pistol that fires rocket propelled bullets? The gun has almost no kick at all, but the bullet hits like a freight train.
How about a continuous laser beam, instead of a Star Wars style pulse laser? Swing it across a room and cut everone in half.
Acid bullets.
Monofilament bolos.
Some sort of electrical pulse that scramble neurological circuits: an amnesia gun.
Ice gun. Pour in water, and it freezes into sharpened spikes. The gun then fires them via a spring. Now, the ice itself might not do a lot of damage, but you can freeze all kinds of chemicals into the ice, and they’ll melt in the body…
Similarly, you could use a squirtgun-type weapon filled with DMSO and your chemical of choice. The DMSO will carry the chemical right through the skin and right into the body.
There’s also the tangler: fires an expanding, sticky web that surrounds the target and constricts. Make it out of monofilament for extra nastiness.
You could also use a pleasure gun. It could fire a beam that overloads the pleasure centers of the target. Being stunned never felt so good.
Of course, none of these things works very well against a non-living target. For that, the gyrojets, railguns, and diskflingers that previous posters suggested would work nicely. Though, depending on how close you want to adhere to real science, a gun that supresses the charge on the electrons of the target could be quite destructive…
I knew I could count on the Dopers. The flechette rifle is a definite, and I’m considering adding a second weapon, if I can narrow it down from the other suggestions. Thanks, folks. 
I have an idea. It’s somewhat borrowed from normal SF concepts, but the details are mine.
The weapon resembles a largish handgun, a bit larger than a Desert Eagle. The barrel is thicker on all sides. There is no ejector port, instead, a vent is placed here. Air is sucked into the vent and compressed. Power is applied from either a power source in the grip, where a magazine would normally go, or by a belt-worn generator that plugs in (Your choice)
This ionizies the gas. Magnetic coils in the barrel then pull the plasma through, similar to a railgun.
The gun has several drawbacks, though. First, the range isn’t as good as a standard firearm(the bolt begins to dissipate around 800 ft downrange, becoming ineffective around 1100 ft. It is acceptable for close quarters engagements, however. Second, the bolt mostly causes surface heat damage. Lethality is similar to a standard firearm, but the affected area on the torso is larger. Body armor either melts or burns, and several shots may be needed. Third, the fire rate is slowed, about 1 shot every .60 seconds, as the gun has to compress more air.
The gun’s greatest advantage is “unlimited” ammunition, as long as there is a power source.
There is a variant that resembles an H-K MP5. This version uses cartridges of already-compressed gas. Cartridges are loaded in a magazine, similar to an automatic weapon. It has a faster fire rate, slightly slower than an average SMG. The range is extended, as is overall damage. Specifically, it is more penetrating. This variant always uses a belt-worn power source, and is bulkier than a normal SMG. The advantage is ammunition is very light, and ludicrously cheap. A device that provides atmospheric gas can be fitted, giving “unlimited” ammunition at the expense of fire rate. Range and power are unaffected.
No weapon of the type I’ve outlined above works in vacuum or low pressure situations. Either there is no air to compress, or the low atmospheric pressure causes the bolt to dissipate rapidly.
The bolt’s color is determined by the atomic emission color of the gas comprising the bolt. Cartridge-based systems can be fitted with a gas that produces light in the infrared or ultraviolet spectrums, effectively rendering the bolt invisible to the naked eye.
If you really want to get radical, the weapons I’ve outlined can also fire normal ballistic ammunition, but I feel this is pushing the envelope. Just outlining all the options…
The quantum singularity projector.
Take a railgun and pump it full of hydrogen. The coils separate each individual hydrogen atom and accelerates them at 99.999…%the speed of light. Due to Relativity, foreshortening will compress the atom nearly flat, thus it collapses under its own mass to form a mini-black-hole, which wil suck up and eat anybody its fired at!
Vortex maker would look like a Hoover tornado, but the tornado would reach out and suck in a mugger’s gun.
I’m rather proud of the handgun I created in a story I left unfinished a couple of years ago. It’s a pistol that electromagnetically shoots crystallized iron flechettes at variable velocities. The standard firing mode quietly shot a loose pattern of the shards at speeds just under the speed of sound, which would cut right through most soft targets. The higher-end models also can fire a much tighter and smaller group of the shards at several times the speed of sound, which can penetrate armor and inflicts catastrophic damage on soft targets from hydrostatic shock. If you have the right implants the pistol can form a link with your nervous system when your skin touches the grip, which enables it’s smart-gun features - it can acquire targets with data from it’s own radar and the optical centers of the user’s brain and aim itself by controlling the muscles in your arm.
I’m probably not going to finish the story and I don’t care if you use it, if anybody comments on it just tell them you got the idea for the gun from me.
(this is based on a short story by (I believe) Asimov)
How about a weapon which plays funky space-time tricks on a projectile so that the projectile doesn’t change it’s velocity, but rather changes its frame of reference (presumably to the center of the local galaxy), and thus appears to accelerate instantaneously to some incredibly high speed. This would have the interesting drawback that it could only be aimed in one particular direction. Or maybe it would have 3 or 4, depending on whether it’s going to hook up to the frame of reference of the solar system, galaxy, galactic cluster, or universe, with varying degrees of destructiveness for each one.
Another idea: build a wormhole with two ends which can be switched on or off. Then attach one end of the wormhole to the end of a pistol. Drop the other end into the Marianas trench. Presto, the world’s most powerful water pistol. For more fun, drop the other end into the sun 
Nothing beats Iain M. Banks’ Lazy Gun. The weapon with total control over local reality and a wicked sense of irony. Practically idiot proof as well.
Reality Alert! Simmons didn’t invent this. They actually exist.
Scary, huh?
How about the glass-needle gun from Stanley G. Weinbaum’s “A Martian Odyssey?” You pull the trigger, a pellet drops into the water contained in the handle, generates an explosive discharge of gas that propels a substance-coated glass needle. Seems to me that they’d be unstable in flight unless they were travelling fast – faster than a carbide-type reaction would toss them.
a stungun but not an ordinary one , you point and press the button and it incapacitates the target by stimulating their
brain pleasure centres so they fall over all whoosy al la Major orgasm or weed smoking session , course useful as a weapon it can also be used for recreation too
The Laser Dazzler is a non-lethal weapon that is kind-of science-fictiony.
The Gyro-Jet that Tranquilis mentioned is a real gun, too…its big drawback is that if you’re within a few feet of the gun, the projectile is moving too slow to hurt you. You can actually put your hand over the muzzle and stop the projectile from leaving the barrel! They were introduced in the late '60’s and just didn’t catch on…
Depending on your character’s environment, and what the purpose of the weapon is, an electromagnetic pulse (EMP) gun might be useful…it would fry any unshielded electronics it was pointed at…computers, battle droids, spacesuit controls, power cells…and have no effect on living beings. It would need a pretty efficient power source, and the amount of power available would affect its range, (and he/she/it wouldn’t want to use it wile aboard any spacecraft or other vehicle) but it could be a fun element in the story.