Just as a little sidenote: The Germans attempted to try and re-kit the MP40 into a high-capacity submachinegun in response to the Soviet doctrine of the heavy usage of PPSh-41 and PPD-40 submachineguns. The MP40/I with a dual-magazine system that worked by sliding one magazine to the side, latching the other in place in the process was the "solution" to the problematic issue of the MP40's viability in sustained firefights. They didn't perform very well.
if i remember correctly one of the reasons was because it disrupted the balance of the weapon.