Air pressure powered dart guns are in the arsenal, for sure, but mostly because they can deliver some kind of machinery downrange without breaking it. Into a living target if they’re really nasty ones, but for lots of other types too - lights, cameras, microphones, chemical probes, beacons, GPS trackers, guidance markers, tiny little robots, and the list goes on. They’re considered logistics weapons, as opposed to anti-personnel or anti-materiel weapons.
But at their low velocities, the darts don’t create enough friction to slow them down much, nor have enough energy to deliver much damage to the target.
A .22 bullet has muzzle velocity around 335 meters/second and weighs three grams. It delivers 168 joules of energy when it hits. That’s the amount of damage it does. And it has about a thousand g-m/s of momentum. That’s how hard it pushes you and the target apart.
A fictional weapon fires 0.1 gram pellets at 2000 meters/second. That delivers 200 joules of energy, which is very comparable to the .22 shot, but it has only about one-fifth the momentum. From the perspective of the shooter, you get one-fifth the kick. The target gets about the same amount of damaging energy if it hits at that speed, but like the shooter, only gets about 1/5 the kick. The damage is in the shock wave that spreads from the point of impact.
The big difference is friction. That one-tenth gram projectile at the same level of kinetic energy (damage) has one-thirtieth the mass, one-ninth the surface area, and six times the speed.
I won’t bother you with the math, but the way it works out, that tiny pellet is delivering more energy into the air between the shooter and the target than the much larger .22.
Enough, in fact, according to my math, to vaporize the pellet about fifty meters out. That’s the weapon I call a ‘needle gauge’ fletcher. Needlers that fire hundred-round bursts make the air incandescent with heat, and the projectiles themselves just plain vanish before they get very far.
The more ‘conventional’ arm for my soldiers is the ‘nail gauge’ fletcher, which fires quarter-gram rounds at a similar range of speeds. That’s about 440 joules, so the damage per round is about a fifth of what you’d expect from an AK-47. But the ammo is tiny and they can carry lots of it, and the rate of fire is MUCH higher than five times an AK-47. Some of the ‘enhanced’ soldiers can handle burst-fire settings up to 1500 rounds, but hundred-round bursts are a ‘normal’ setting.
By the way, 100 rounds in a half-second burst is the frequency of the note G3. 1500 rounds in half a second is four octaves higher at the note G7. That’s the pitch their weapons shriek when they pull the trigger. Hearing protection is advised.