I sort of like the variation on the gauss between prime and c. Not too difficult to make up some good excuse for why it is so. I will definitely agree that the boxy UAC20 looks very weird though.
I shudder to think how fast it would fall of if it was bigger however. That UAC20 variant needs a bit more prolonged engagement window then the LBX one, so it ends up exposing that main weapon longer. The nature of close combat is also such that there will almost always be an enemy that can target that arm, and it does present a very tempting target. Still, it does look damned weird being in that little box. I mean, with that big a projectile it would probably even be hard to fit the firing mechanics in that thing.
/Merf - Bag-in-box, now with UAC20 flavour
