Also, guns in the ME series don't shoot plasma - the closest you get is the Geth Javelin shooting ferrofluid. They shoot miniature pieces of mass through a small mass accelerator powered by mass effect field. Each gun has a block of an unknown metal (I'd guess steel or titanium, unless a better material is available) and each round is a very small shaving of that block. Shotguns shave off many smaller pieces, and the Eviscerator shotgun shaves off blade-edged pieces. To my knowledge, a single block can last a gun an entire war, but I hardly think you can go a gun's entire life without replacing the mass block.