I have always suspected something like Johnny's quote, all the apocryphal stories not withstanding. Having two different loads that will chamber in the same gun would create a logistics nightmare that no military organization in its right mind would want to contend with.
If you have a SMG load that is too hot for the pistol, there will always be some damn fool using it in the pistol, no matter how many warnings to the contrary.
What's the point of a hot SMG load? Easy to design the SMG to operate on the standard pistol load and the usual bugaboo with a SMG is keeping the recoil within reason so that you will be shooting at the opposing troops rather than your own aircraft. The second design problem is keping the cyclic rate within reason, and here a milder load helps also, as a hot load will cycle the action all the faster.
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