So it's an ML-16 with a shorter barrel and perhaps a battery mounted in the rear then...
To me it looks like [V] took the ML-16 and turned it around so that the back end serves as the barrel, and then they added some other details, namely a cover to make it look different.
And why do they still use bullets? Are they some kind of light slugs (mini-Maxim)?[/color]
The whining sound you hear when they open fire could be coming from it that big gun that the woman marine holds. I'd associate that with an autoloader or something for a minigun (that said, I know nothing much about real guns). After she kills the big Shivan, the whining sound stops, so maybe it is generated by that gun?
But I've just been watching the cutscene again. Does the ML-16 type weapon have a slow or high rate of fire? When you see the woman marine firing her gun before she shouts 'DIEEEE!', you can see the barrel firing at a slow rate.
IIRC I think someone on this forum in an earlier thread said the Shivan that exploded was possibly hit by an AP grenade. Maybe the weapon she has has a sort of underslung grenade launcher? That may make some sense... But the expoding Shivan could be explained by a natural self-destruct mechanism Shivans might have for use when they die.
Maybe it's a big shotgun? The short-barrelled weapons the other marines use seem to of the be of the rapid firing machine gun type. Using a shotgun would make sense if you assume that a ship's hull could be easily penetrated by solid slug type ammo (Shivan fighter hulls, at least, aren't that strong, not sure about freighters). But then, not all the marines are using the same weapon.
Just some random thoughts.