Stop me if you've heard this one...
"So both those models are built from the same kit?"
" Yeah, both are carnifexes, this one has the rupture cannon and the other one doesn't."
"So that means... You've got another one of those big dingus cannons left over on the sprue. That you aren't using. That I could loot."
Thus, the most disgusting mek gun to date. Some truly degenerate mek had his Grots hack the 'goopy zappy kannon' off a dead bug. Then he hooked up a bunch of tubes and pumps, added a barrel of toxic industrial waste, and somehow made it work. Even if the gun smells awful and constantly oozes pus from various orifices.