Firstly, if this sounds similar to DK's Micro-Missile Battery, it's because I thought of this awhile ago but never posted it. DK's weapon just reminded me.
My idea is for a multi barreled missile launcher that fires lots and lots of tiny, unguided mini missiles, sort of like a cross between a missile launcher and a machine gun. Each missile is somewhere between 3 and 6 inches long and less than an inch wide. They travel much faster than a G5 rocket but they're still a long way from being instant hit. At long range you'll still need to lead your shots quite a lot.
They have a small fragmentation warhead, but due to the tiny size of the rockets the blast radius is small and the damage from it is low. Most kills are scored by hitting an enemy directly with a missile. It takes 2 to 4 direct hits to kill an enemy with full health.
Because they are so small the missiles are less than reliable. Their warheads will always work but most of the missiles will deviate from their flight path after flying for awhile. In game turns this means that missiles fly straight for a short distance and then begin to change direction in some way. This might involve gradually flying to the right rather than straight ahead, or it might involve corkscrewing wildly in the general direction of the target. Some missiles might suffer drastic failure and shoot off at a 70 degree angle, while others might run out of fuel and fall to the ground. In particularly rare cases the missile might even work perfectly and hit exactly where it was aimed
While crazy, inaccurate missiles might sound like a disadvantage, they are actually the gun's main strength, because they allow the user to saturate an area with a massive hail of missiles that hits not only where the gun is pointed, but also most places within 8 meters of it, depending on range. The reason the gun can do this is because it fires very, very fast.
Primary fire: Shoots missiles one at a time in quick succession. I imagine the rate of fire would be similar to the M925. This fire mode would be used for conserving ammo, or killing a single enemy at short to medium range.
Secondary fire: Spews missiles from every barrel at once. The gun would have an ammo capacity of about 60 rockets, and using this fire mode continuously would empty the magazine in 3-6 seconds. This would be used to cover an area in rockets or to ensure a kill at close range, but would waste ammo quickly.
Physically the gun would look like a huge heavy cannon about the size of the minigun. It would have a carry strap like the minigun and a handle on top of the weapon for the left hand to hold. The end of the gun would have 12 to 24 holes arranged in a circular formation for the rockets to fire out of.
All in all the weapon would look like a large, man portable rocket pod of the kind used on helicopters. Here is a picture: http://www.aircraftresourcecenter.com/AWA1/001-100/walk072_Apache/images%20Gary%20Fairfull/apache01.JPG
Comments, suggestions, criticisms?