Gyrojet cannon? Fires difficult to aim projectiles that accelerate over time, long range but due to their slow initial speed they're really easy to dodge? Explosive damage, I assume they'd have to actually be tiny rockets but would be fitted in a small ballistic slot and have comparable ammo (120 or so? Fire rate of 1.5 per second, or thereabouts?)
Hmm... It is difficult to come up with good ideas for small ballistic that haven't already been done, isn't it?