As far as I know, the submachine gun has a somewhat different and simpler operating principle. The assault rifle key invention is piping gas from quite far dow

As far as I know, the submachine gun has a somewhat different and simpler operating principle. The assault rifle key invention is piping gas from quite far downstream the barrel to a piston that pushes the receiver back and makes it pick a new round. So you get delayed pushback. In a submachine gun with immediate receiver recoil you either lose a lot of gas or have to make the receiver heavy. That's also why the AK47 looks like it looks, it has a "twin barrel" part way, that's where the gas piston is.