A couple of ideas
do not get to full velocity until they are 60m from the muzzle
you could start them with a traditional charge, like smokeless gunpowder(nitrocellulose). It would leave the barrel slower then typical round but still much faster then a gyrojet alone (see #2). For example, it's probably a large caliber and so you don't want so much of a charge that it has a big kick. Maybe something like 1/4 to 1/3 of the max speed. This is just one thought.
You could use a magnetic acceleration method, like a coil gun. Maybe they don't have good enough tech to make full on rail guns, but maybe enough to get up to the 1/3 to 1/4 speed. This would be my choice if the energy requirements can be met.
As others stated you could simply use a faster burning solid rocket fuel or even compressed air.
- Depending on the tech level you could also add guidance to it. This could be laser guidance where it rides a laser pointer beam. It could be heat seeking, motion seeking, or whatever your tech allows
- You could use a discarding sabot round. This would let you put fins on them, basically the sabot is just some lightweight hi-temp foam/plastic that makes the arrow/flechette shape fit into a cylinder. You could also put a spacer between the traditional charge and the gas exhaust ports, if they need protecting from the starter charge (see #1) if using that method, so per-ignition or damage to the round is not an issue.
Something like this (for the gunpowder starter charge version) but with internal fuel as well. So if you can imagine replacing the solid sabot core with a self propelled flechette.
- ammo is expensive
- This is relative. If the ammo is mass produced the cost would come down a lot. It would always be more then a traditional round (especially if it had guidance). But that is the nature of the beast, a smart bomb costs more then a dumb gravity bomb. It's just the way it is.
For a ideal sniper rifle
You may actually want a slower round. Speed is not the top priority. You really want to keep it subsonic. The problem with subsonic bullets is they are losing speed all the time so they have poor range and accuracy compared to high velocity bullets. They also have less penetration. But they have the advantage of being [more]silent as there is no sonic boom. Most people think that the majority of the noise from firing a gun comes from burning the powder charge when really a lot of it comes from breaking the sound barrier.
So an Ideal sniper weapon (in my mind), would be as follows.
- Boost phase, magnetic acceleration initially gets it started. This is silent, as long as it's subsonic. Maybe it makes some humming noise but much less then a gunpowder charge. There is no smoke, no flash etc. Maybe some sensitive equipment could pick up the magnetic discharge or the energy spike, but whatever.
- cruse phase, using internal propellant. This just need to maintain the speed, or accelerate it up to just below the sound barrier. You would also do bulk guidance in this phase to get it going in the general direction of the target.
- terminal phase, the round would accelerate as much as it could in this phase breaking the sound barrier.
- You could use a separate fuel that burns faster, or if you can imagine having multiple holes with independent chambers, like separate rocket motors then you could just ignite additional "rockets" to increase the thrust. For example it has 6 ports in the bottom and uses 2 for the cruse phase, then it sets them all off in the terminal phase.
- This phase would only be able to do fine guidance because of the speed.
Ideally it would use a solid propellant that burns as clean an clear as possible. I really like the idea of having many very thin rocket chambers. Perhaps a few dozen of them, because then you could pulse the firing of them and have finer control. Essentially it would not need to use the "rockets" the whole time, only to maintain speed an course. the trick here is you cant turn off a solid rocket. You can burn 2 chambers on opposite sides for a split second for low thrust, wait and then burn one or two on the right if you need to turn to the left. Or ignite them all for a quick burst of speed. They would be high thrust but short duration.
By the time it does break the sound barrier it's already down range, making it hard to trace back to the shooter. The slow speed would let you do some crazy guidance, like 90 degree turns, also making it hard to trace back to the shooter. The terminal phase lets you ramp up the speed so you get the penetration ability of a high velocity round while not giving away your position. Of course the round itself could have explosives (shaped or otherwise) or poisons etc.
A machine gun
This would work almost like the sniper type/mode. The only difference here is you would essentially just eliminate the cruse phase. Mag acceleration here would help with the rate of fire and by removing heat (from propellant exhaust) from the gun barrel. The round is outside of the barrel when it's little rockets ignite.
I could imagine a coil gun with powerful magnets that you cycle. To fire the next round you just shove it out of the magazine into the first magnetic field with a physical pusher mechanism. Then you could fire as fast as you can cycle the fields and push new rounds in. This would probably need a good electricity source though, but it would me much less then a full on rail gun or laser. Sort of a composite weapon.
Last thing is because you lose the cruse phase only fine guidance would be possible at these speed. This would make the auto mode better at short range but less accurate, which is what one expects.