...I'd think that a good rocket gun would use a much larger round that allowed for more payload, internal guidance, self-seeking of targets, etc. After all, with little recoil this would be entirely possible. ...
I like that. The original gyrojet was something like 12 or 13 mm diameter, which is kind of awkward for an ammo load but, if there were features that made up for the reduced number of rounds you could carry, they might be worth it. It'd basically be a little 16-gram guided missile, of which 14 grams is missile body and about 2 grams is propellant. Tiny radar maybe, give it fins and a sensor just good enough to detect a person just far enough away to make a slight course correction and turn a near-miss into a hit, giving it a small to-hit bonus. Problem is it's likely to be too small to discriminate between a body and cover - might be defeated by an array of bars. IR's a thought but could be defeated by a heat source. Wire-guided? Useful for sniping if the velocity is kept below the speed of sound but hard to maintain aim in a firefight. So, a sniper rifle. A bit more propellant might let it reach speeds that would defeat cloth, but you'd get that sonic boomlet.
Much bigger than 13 mm and we're venturing into the world of tiny RAM grenades. I understand they've been working on versions as small as 20 mm, though I'm not sure how effective those are. But then, those would be roughly TL 7-8 equivalent; presumably future tech versions would be more effective. Homing RAM grenades would be way cool, though they might render battledress obsolescent if they could also be made to be reasonably powerful.
The close-range power problem of rocket rounds could be addressed by using HEAP heads like the ones used in the snubbie rounds, though fitting that in while also having whatever guidance is needed in there as well is probably asking too much. As an alternative to the snubbie, though, it may have potential. The round reached a velocity equivalent to a snubbie after about 2-3 meters flight, and it got faster after that. Flatter trajectory, longer range, makes it a better option for delivering HEAP rounds, makes it a better sidearm once cloth is in common use, until gauss pistols are available.
At melee range it'd hit with about the force of a rubber bullet, might be enough to set off a HEAP but I don't think you want something going off a meter away from you - certainly not in
Striker, where it has a 1.5 meter danger space - so probably not built to go off at that velocity. Still, hitting someone with something equivalent in impact to a rubber bullet might at least throw them off for a second or two.
An interesting thought is that because the launching barrel doesn't need to be very strong, the launching barrel could be in some rather unusual configurations. Pretty accurate walking-cane gun since it has a flatter trajectory and no recoil. Relatively easy to make a zip gun if you have access to rocket rounds. Relatively easy to make a barrel you could strap to your forearm, assuming you could figure a way to fire it without it accidentally firing at inopportune moments. Come to that, a person in combat armor or battle dress could have a fixed array of several single-shot launchers with HEAP rounds, set to go off at relatively low velocity, mounted on the armor as a means of discouraging a lightly armored mass of attackers from trying to overwhelm him at melee range; risk of friendly fire injuries to others in combat armor would be nil at TL14+ and probably acceptable at TL12-13.
