This weapon would be kinda like the flamethrower, but with different visuals and a slightly different effect.
It would look like some kinde of big, blue-ish leafblower wich spouts out a bright, shining cloud of plasma with electric currents running down the tip of the barrel when fireing.
As plasma is ionized (electrically charged) gas, the cloud could do electrical damage to especially vehicles, aside from the obvious damage gas hotter than the sun's surface would do to flesh and metal.
Ofcourse, this weapon would need some kind shield with a visor, kinda like a big welding mask, to protect the user from the extreme heat.
To be able to fire the thing without doing bigass selfdamage, the user would have to AIM the weapon first, limiting it's effectiveness.
The weapon should be very bulky to house the ultra-compact fusion reactor, wich produces plasma out of
Helium 3 (an isotope of the gas Helium), too. Therefore it'd make you run even slower than the railgun (but not too much).
As plasma rapidly diperses into the surrounding air when not bound by a magnetic field and because of the extreme heat transfer caused by the contakt with increasingly large portions of surrounding mass this weapon would only have about 1/2 of the flamethrowers range.
Secondary fire could be as follows: A high-power capacitator sends a current through the plasma, wich forces all the plasma in the fusion chamber out of the barrel. Effectively a plasma ball wich would disperse after going about three times as far as the plasma flame, costing as much as a whole Helium 3 canister of ammo and doing damage accordingly.
Special function key could turn off the fusion process and make the weapon spray super cool liquid Helium 3 (wich crawls against surfaces and walls) of allmost 0 Kelvin, wich would have 1/3 of the plasma's range, and stay on the floor for a short amount of time, freezing anyone who stays long enough in it's area of effect or gets sprayed directly with it.

You can't forget Miracle Matter's Electro Plasma Cannon either.

Uh, flamethrower style weapon?

Mine is hardly that... In fact it's quite similar to the "Tempest".

I thought you were just suggesting Plasma Weapons in general...

But nonetheless, my cannon would be usable at varying ranges, though further ranges would pretty much be reserved for slow moving targets. So I guess you can say it'd still be a shorter range oriented gun.