If I remember the technical specs laid down for mass driver guns then the weapons in the movie were mass driver fire arms. The definition of maws driver gun, that I remember, is basically a rail gun. An elctromagnet system fires a metal projectile, sometimes close to the speed of light. The small arms in WCM were indeed rail gun type weapons, they used electromagnets to accelerate a metal projectile.
Anyway, the WC4 book dosen't fully elaborate on the actual type of projectile fired from the side arms( It's obviously an energy weapons, possibly laser, but the wounds are a little strange for a laser). Anyway I thought maybe making them Electron guns for Marine Force Recon. So when you shoot somebody with one it short circuits their nervous system, and kills them.
I don't know, I could be way off base here, but hey what do you think?