Lord of the Abyss wrote:
Still; once they leave the barrel they'll start losing velocity, while the Gyrojet will be gaining it. And I'd think a smaller bullet would lose velocity faster, due to having greater surface area compared to its mass?
If you're artificially limiting the terminal velocity of the round to subsonic, then the rocket acceleration of the rounds will be likewise limited. Your motor is just a weak sustainer rocket, which makes it even less
worthwhile than if it were a powerful rocket, because the gain in energy and speed is less.
See, the problem is that you're saying "subsonic rounds are good, and here's how Gyrojet improves a subsonic weapon," rather than working the other way round and saying "what are the problems with subsonic munitions, is it worth the effort of building them at all, let alone improving on them?"
That is correct. Also, a bullet with a smaller mass would for the same amount of kinetic energy have less momentum than a heavier bullet fired slower. And as far as I understand its the momentum that makes those neat soft tissue wounds and helps penetrate armor.
That gets extremely complicated- and the point of the exercise is to increase
energy while holding momentum constant, not the other way round.
For example, Napoleonic muskets were around .75 caliber. The improved blackpowder rifles of the late 19th century trended down toward .45 to .50 caliber; modern rifles fall into the .22 to .30 caliber range. What's been changing, by and large, is the muzzle velocity- big slow bullets replaced by smaller faster ones, which means you can carry more ammunition.
Another advantage that gyrojet rounds would have for a small-arms coilgun is that, thanks to their angled exhaust vents, they would have spin stability. Since a coilgun probably couldn't spin its projectiles, they would otherwise have to have fins for stabilization and sabots to hold them securely in the barrel, like an APFSDS anti-tank round. Finned projectiles would only be stable if fired in an atmosphere, but gyrojet rounds could potentially fly just as straight in a vacuum.
...Why on Earth would you need spin stabilization in a vacuum? There's nothing to make the rounds tumble.
The problems with subsonic rounds are delay, projectile drop, and damage potential.
For subsonic rounds, it seems a gyrojet would be a better choice than a bullet. Delay will be less, as the gyrojet could stay near the speed of sound for longer. Shorter flight time means less drop, and faster speed at the point of impact means more damage from the same mass of projectile. And they can have a lighter gun anyway.
Those advantages over bullets would also carry over to supersonic rounds.
Though is it enough of an an advantage to be worth the extra cost of making gyrojet munitions ?
Probably not. If they looked like they could be worth it, then I'd expect someone to be making them in sufficient quantities that we would have real world data about how well they work in comparison to modern firearms.
The problem is that as long as you're committed to keeping the bullet subsonic, you can't
make it all that fast or powerful for a given caliber. You just can't- Isaac Newton is working against you.