Cryhavoc101
Donor
The earliest assault rifles used shortened case versions of standard large caliber rifle rounds. What if they had gone the other way, and used lengthened case or overpressure pistol rounds to create heavy pistols and submachine guns? Would 1930s and 1940s metallurgy allow for semi-automatic pistols and submachine guns to deal with the increased pressure and heat generated by such powerful loadings?
In Tony William's "The Foresight War" the British throwback has the 'British' develop a good copy of the select fire Solothurn SI-100 SMG chambering in 9mm x 25 Mauser export and a copy of the Browning/Colt Pistol design chambering the same
The intention was to develop a low risk controllable automatic weapon for the British armed forces that was useful at most battlefield ranges.
Another thing I once read was the 9mm Para Ammunition issued to Australian units armed with the Select fire Owens SMG was due to an accident at the ammunition factory effectively overpressure ammo or P+ aka 'Hot' ammo and was therefore fired at a higher velocity than normal 9mm para ammo of the day.
This along with its very rugged reliable design and full auto controllability made it a superb jungle fighting weapon.
So perhaps create a Owens 'Machine carbine' firing a 'hot' over pressurised 9mm x 25 'Mauser export' round.