First, the Luger is recoil operated, pressure has nothing to do with it. As a loose rule of thumb a load with a heavier bullet will produce more recoil.
The 9 mm and 7.65 mm Lugers had two different recoil springs. Unfortunately, shooter grade lugers tend to have out of spec springs because they have been doctored by the unknowing according to one or another urban legend. It’s been my experience that if I open up six shooter grade Lugers, I will find seven different recoil springs, none of which match spec. The first question is, do you have the correct spring?
The Fiocchi stuff is seriously under loaded. It’s quite possible that the breech block is not going all the way back and is picking up the next round by dragging on the case or groove rather than getting behind it and pushing as it is supposed to do.
The magazine is always a suspect. Vintage magazines have “history”. They are often not up to par. Look at how thin the lips are and consider what happens if you drop one on its head.
Modern 9 mm ammo is loaded shorter than the original and this can be a problem. I’ve not hear of it as a problem with the 7.65. The bottle necked case tends to feed better. I used to shoot semi-wadcutters and full wadcutters from a 7.65 Luger.
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