This is a really good space weapon, I love the detail. So I assume you envision the the plasma projectile moving slower than light.
I can already hear the players debating it merits.
Reasonable Player "It takes up too much space and has a low rate of fire"
Excitable Player "Yeah but only need to fire it once"
Meta-Player "The GM isn't going to let you take a bad guy with one shot, I don't care how big the gun is"
I like it, in campaign I can certainly see it setting the tone for the conflict. But the hard to use potential and the possibilities for dramatic mistakes makes this a wonderful tool to hand your PCs. Really a solid item.
One note, i think defense is spelled with an s not a c