So.. Shouted out my concerns on the Discord about the SMG/SGG debate. Really, came down to this: They're both likely to be really damn hard.
The SMG-12 was originally Theoretical; at least partly because we didn't have anything other than Bolt-Action and a Gatling Gun. SMG's are also a decent bit ahead of time. This hasn't changed. I'd expect that the SMG-12a would either hit with Very Hard (-2 to rolls) or Theoretical (-3 to rolls!) as well, due to trying to get that fully automatic open bolt action with a revision, when once again we have literally 0 experience with the things.
The SGG-12, on the other hand, calls for a quite frankly massive miniaturization of our current models of Gavrilium engines. Our current smallest engine is the one utilized in the Lifting Suit, weighing in at 10kg, and way too big to feasibly mount on an SMG style gun. I don't have a good guesstimate for difficulty, though I wouldn't be surprised to see a Very Hard here as well. It's just a really big step on miniaturization.
Now, I myself am sticking with the SGG; partly because I feel that rotary autocannons are neat (yes, I know it's a fairly shallow reason, but I do judge both to be roughly as viable as the other); and this is the setting where such a thing could flourish. Weight isn't too much of an issue; we can easily adapt the Lifting Suit for combat use. But still, this is both a decision of stylistic choice, and one of what side benefits we want to get out of the revision: Engine Miniaturization for the SGG-12, Open Bolt action for the SMG-12a.
With that, I rest my case; whatever your choice is, make it. Let this revision be decided via Trial by Votebox.
Alternatively, we find something else to revise and just go with a Shotgun, or do the Power Armor + Sword thing Madman was talking about. That would probably be less reliant on good rolls.