deterrent weapon
An ideal deterrent weapon should have the following minimum qualities:
1) non-lethality
2) stopping power
Good examples of such weapons in science fiction might include a stasis gun, a freeze gun, a sleep ray, or a passivity field (in which the victim loses volition). Basically. I want a weapon I can fire at anybody to stop them utterly, but which will result in no lasting harm if fired in error.
Currently, members of law enforcement have some mediocre tools for this problem. Tasers, bean bag guns, rubber bullets, and fire hoses are a few examples. Raytheon has also developed a microwave based “pain ray” that seems pretty effective: http://en.wikipedia.org/wiki/Active_Denial_System
One problem with bean bag guns and rubber bullets is the following: what is effective at one (longer) range is lethal at another (shorter) range. Conversely, a muzzle velocity that is effective and non-lethal at short range is ineffective at longer range.
One solution I’ve come up with for this problem is adaptive ranging. We have the technology today to adaptively range a target and program the muzzle velocity even as the trigger is being pulled. One can imagine switching quickly between near and far targets and the gun automatically deciding what power to use.
In Dungeons and Dragons, a good magical equivalent of a non-lethal weapon is the Web spell. As its name implies, it involves shooting a mass of sticky webbing at the target. Spiderman is also famous for doing this.
With adaptive ranging, a “web” gun could launch a large packet–say, one to a few pounds– of “webbing” at the target with stopping force regardless of their distance. The webbing would have a quality like hot glue–liquid on impact, followed by quickly drying and hardening into an immobilizing mass.
Difficulties I can immediately foresee are as follows: your gun would change its weight (and therefore handling) drastically with each firing; in order to maintain a constant liquid mass, the gun would either have to have a heating coil, or, more likely, high pressure contents.
Alternatively, one could imagine an “air bag gun.” Air bags work by igniting a chemical reaction that produces a huge amount of contained gas very quickly. One could fire a cartridge at the target that would explode into an airbag immediately in front of the target. This would be loud, sight-obscuring, and momentarily impeding.