A Rocket Rifle that fires low powered accelerating rockets at a moderate initial velocity which will eventually accelerate to extreme speeds,
another feature of note is that the rounds deal greater damage at higher velocities.

Edit:
Low velocity high acceleration version added
External Dependencies removed

i think the idea of gradually accelerated a bit flawed,bullets go out at extreme speeds when fired then why not rifle launched rockets?It would blow up the pipe if it doesn't have an exaust thingmabob

I get a crash*.

I think your idea of how impulse works is a bit flawed. Impulse = Force x Change-in-Time and is imparted on both objects. To accelerate a rocket* to the same speed as a bullet^ in the same amount of time you need a much larger force (because f=ma and we have a larger mass and want the same acceleration). If you decide to do this, that much larger forcee then gets applied to the soldier firing the weapon's shoulder. Considering the amount of recoil you get accelerating something 300 times lighter to the same speed (enough to throw off your aim) the much larger force would injure the soldier firing the weapon if not throw him clean off his feet, which would prevent sustained fire and undermine the reason for constructing an automatic open combat rocket rifle, which is to blow the living hell out of whatever you shoot it at. In order to get a sensible impulse applied to your soldier, you apply the much larger force over a longer period of time, while the rocket is in flight. Its initial velocity may be lower, but you get to keep your soldiers with their bones intact and their arms attached, which is a worthwhile trade-off.

* rockets are typically ~3kg for arguments sake, a lighter rocket from maybe an RPG-7. (If you were going to make a rifled, (semi-)automatic infantry rocket-based weapon you'd strip this down to lower because you most likely lose your ability to fire recoilless, or at least se7en's design does, but the rockets here look roughly RPG sized if we're going purely by the ppm scale)

^ typically 10g


@se7en that sprite is delicious, it doesn't matter if it's a mash-up. So hilariously tacticool.

Update:
Lower velocity, higher acceleration version added
External Dependencies removed (Suprised no-one complained about this) seriously I need to stop doing this

Thanks, I really like it too, kinda went overboard a bit though, not that that's a bad thing (if i win i miiight make the next theme tacticool in honour of this...)
Something that may be of interest is that this mod was started yesterday, and only took about 2-4 hours of coding/testing overall

Y'know what'd be kewl? If the gun had tiny, one-pixel fins that turned because of the rifling. Just make the round animated.