Ideally, you want to set some magnetic stabilizers on the end of the barrel (traditionally reperesented as south/northpole prongs like a
tuning fork (
you should recognize them) to achieve fairly basic performance you've outlined.
If you don't, the round density will be low and performance far closer to a vapor than a cohesive round (think flamethrower).
The notion of a gas-tank is also important unless you're combusting ultra-pressurized air (breaking the hydrogen from the oxygen and nitrogen chemically to get your plasma -- Which is a very immature technology almost noone has looked into (and hence any early "breaker/distilleries" will be quite unwieldy) witht the exception of the AMX-102 (which uses purified water, not air) so you really do need a fuel/munitions tank.
Think of it like a fuel tank full of non-volatile materials so you can armor and ultra-compress it so it's at something like 2000PSI (appr. 45lbs per cubic inch, using maybe 0.3 pounds per round you fire).
It might be worth integrating this into the rifle or having multiple tanks or even mounting it into a shield assembly or those thin rod-like tanks you'll know from Neue Ziel or Hi Nu.
If you want to mess with EM shielding, you'll be using plasma which has been electrically excited to carry an electrical charge which when it hits the barrier will disrupts it (perhaps not immediately).
The science behind this (if you're curious):
That disruption I mentioned deals stress on the shield generator manifold which has to cope with the backflow - sort of like pouring a different fuel into an engine so it has to work harder alongside the regular mixture, running that "bad fuel" out until the engine can return to normal performance.
Working harder makes the manifold get very hot and it may need to go into emergency cooling or the charge may destabilize the field - to carry our metaphor over, the bad fuel creates a bad compression in the cylinder it wasn't built to handle and makes the engine either stall or blows the cylinder.
I hope that helps