Like the idea of the Magpul folding SMG. But it would be Class III, and not available to ordinary people, even if they made them.
I do have the KelTec SUB-2000 folding carbine, and it is an amazingly compact and sturdy weapon. Not a military weapon, but it would make a great piece for tankers, artillerymen, and rear-echelon troops.
Oh, they're quite available, but as the PP-90 Pistolet-Pulemet obrazets 1990,/ ПП-90 Пистолет-пулемет, or * Machine Pistol model of 1990*. And they're in the 9mm Makarov chambering.
Manufactured at the Russian Tula arms plant, there are also 9mm Parabellum versions, a selective-fire variant, a suppressed version, and the *Bizon* spinoff from the competing Izhmash firm that uses a 64-round helical-feed high-capacity magazine.
But no, they're not particularly suitable for tankisti, Артиллерия, or MTO-voennii, who need real rifles in the event they come under fire from enemies so equipped. It's a cops and spooks gun, and also useful for security purposes where the sight of a *regular* weapon might spook the horses...and sheep.