Contact with
anything robs a plasma of it's energy very quickly. And anything that can actually contain enough power to create the plasma is probably better off used as the weapon directly.
Also, in terms of actual damage, good energetic plasmas are a pretty close approximation of what would seem like a hard vaccuum to you or I. At least in terms of in use for a atmospheric/infantry weapon.
Although a thin conductive plasma conduit created by a UV laser can shoot a lethal electrical charge in a very straight line.
Kinetic energy seems "primitive" but it's insanely efficient, at least as compared to energy weaponry. A particle beam is almost impossible in atmosphere, save neutrons, but making that many guided neutrons is darn tough, them being well... neutral and all.
Caseless ammo, gasses or liquid fuel powered projectile weapons. (think a cordless butane nail gun) or electromagnetic are probably the most reasonable personal weapons for the forseeable future. Solid-state tactical lasers in the kilowatt range are getting ever miniaturized, there are systems that can fit on a HMMV or small armored car, or even in an F-35 prototype. One that's the size of say a shoulder fired missile/recoiless rifle tube, or perhaps a large broadcast TV camera is concievable in the next 20-30 years, however the power supply is the rub.
Something that was backpack sized that could power such a laser for a reasonable number of shots would revolutionize everything else too. Cell phones that run for a year, laptops a month, "batteries" you could run your house off of etc. And any device with that energy density would also be a halfway decent bomb if it malfunctioned.
