first, a list of HE damage of various small arms in the game. They are calculated/guessed using known HE damage of various squad.
(non sniper)Bolt action rifle: .10
m1 carbine: .12
semi-auto rifle(garand, svt, g43): .15
mp40, ppsh43, stg44: .30
ppsh41, thompson(?): .38
the stg44 fired a intermediate cartridge, more powerful than a typical pistol cartridge, but weaker than a full rifle cartridge.
This mean that the 7.92mm kurz would still be too powerful to be fire accurately on full auto. Even today, an assault rifle would still be fired in single shot for most firefight, the lighter recoil being use to facilitate single shot. The full auto setting is only used for suppressive fire (ie not expected to actually hit).
the stg44's HE damage should be closer to ~ .225, a 50% increase over the semi-auto, similar to how the semi-auto is a 50% increase over the bolt action.