>On an SMG? Are you high?
Not that anon, but he's right.
Compare the weight of the gas-operated Type 79 submachine gun (1.75kg)
https://en.wikipedia.org/wiki/Type_79_submachine_gun
to the Soviet PPS-43 submachine gun (3.04kg)
https://en.wikipedia.org/wiki/PPS_submachine_gun
or to the Yugoslav M56 submachine gun (3kg)
https://en.wikipedia.org/wiki/M56_submachine_gun
…and think also that a gas-operated SNG can be designed to fire from a closed bolt in semi and open bolt in full-auto, which would give a significant accuracy advantage.