Lolwut. Converting a solid slug into a plasma would A) require a huge amount of energy, B) reduce the weapon's range and damage (because a puff of vapor, even superheated ionized vapor, just doesn't hit as hard as a solid object), C) require bulky, energy-hungry cooling systems to keep the weapon from melting... I'm sure there are other problems, but those are the three that immediately occur to me. I know, I know, stop applying real-world physics to a game, but it still bugs me.
The machine spirits take care of all of it you stupid heretek.