>>64066921> Wouldn't that just be a carbineI guess, but "carbine"'s not a very meaningful term these days - now that we're past the days of service rifles with like 30-inch barrels, basically all modern self-loading rifles are carbines, and of course semi-auto civilian versions of SMGs are referred to as pistol-caliber carbines.
I would say that there's definitely some utility to distinguishing a gun that fires 9x19mm from one that fires something like .44 Magnum (or rather a rimless cartridge with equivalent ballistics, since regular magnums might not work that well in a magazine). So, basically I'd interpret "heavy SMG" as "something chambered in a cartridge between typical pistol calibers like 9x19mm and intermediate rifle cartridges" (an intermediate-intermediate cartridge if you will... defining things as being between other things doesn't seem to be working out very well).
SMGs have historically been limited by pistols - militaries adopted SMGs in the caliber they were already using for their pistols, but SMGs, being larger and bulkier weapons, could handle somewhat more powerful cartridges. If you "liberate" SMGs from the limitation of compatibility with existing logistics, you could make them a lot more powerful, and more competitive with subcompact variants of assault rifles (which, using cartridges originally designed for 16-20-inch barrels, are of course wasting a ton of their energy - rather than having it propel the bullet, it just goes up in a nice big fireball at the muzzle).
But that's unlikely to happen. Perhaps if typical rifle cartridges become way more powerful, and thus completely unsuitable for use in compact variants in the Krinkov/CQBR form factor, there might be new developments to fill in that gap in capabilities for special forces and police units, but I don't really see everyone following the US's example with 6.8.
Maybe in the cyberpunk future where we're having to fight Terminators, I dunno