Got a whole kilo on the mp5 lol

tbf, that's just WW2 SMGs in general - the MP40's nearly 4 kilos, the Sten's about the same as this one, the Thompson (which is in .45 admittedly, but still) almost reaches a whopping 5 kg, and even when lightened in the M1 variant is still heavier than the Garand

simple straight blowback is just heavy, in practice SMGs seem to only become noticeably lighter than assault rifles when you've got fancier systems of operation, whether by gas or some form of delayed blowback