I have run into this before. It doesn't really add up and is one of the reasons for starting a research project into the 7.65 mm Parabellum cartridge.
Just as a back of the envelope calculation, NO recoil operated pistol of any make or model should be expected to work with a stock recoil spring and a powder charge 20% below nominal. For the 7.65 Parabellum this would result in a velocity around 800 fps and an impulse around 63% of nominal. (Admittedly a very approximate calculation.)
No doubt there are a few pistols out there that will function with this sort of load, but they are the exception to prove the rule. A pistol could be made to work this way by choosing a stock recoil spring that is lighter at the expense of increased stress and wear with the nominal load.
Perhaps in time, further research will shed some light on this point.
I can make my Ruger with 40 S&W upper work with such a wimpy load by using the 7.65 Parabellum recoil spring, but a standard 40 S&W load with this spring might blow the slide off the back of the pistol. I also have a Luger in 7.65 that currently has a very wimpy recoil spring in it.
Note that the Borchardt had a recoil spring adjustment that allowed it to handle a wider range of loads. The original intention of this adjustment was to tune the individual pistol to the standard load and compensate for variations from one recoil spring to the next.
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