Fiocchi in .30 Luger is too short to properly feed in some Luger actions. It is also too weak to correctly operate many Luger actions. Prvi Partisan ammo is longer and a bit stronger. The Luger action is intended to be a balanced system and will not operate correctly no matter what the mainspring strength if energy from the round is below the required level. Lugers are robust and IMO Prvi will not damage a mechanically sound Luger.
I can tell you from experience that some Lugers cannot be made to operate correctly with shorter/weaker ammo by weakening the mainspring. I once tried to make a Luger cycle with Fiocchi by weakening the mainspring; I finally got it to feed but the mainspring was then so weak it would not return the toggle to battery. With a stronger mainspring and Prvi it functioned perfectly.
I personally would not adjust a mainspring until I had tested a .30 Luger with both Fiocchi and Prvi. When I find I must adjust a mainspring, I start with one that is too strong and progressively weaken it by removing 1/2 coil, re-testing and so on until the spring will reliably operate the pistol with the selected ammo. Tedious process perhaps, but I find it satisfying to return a Luger to proper function.
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