I have had similar problems with a Finnish M23 luger, and found that the problem was not too strong of mainspring, but rather too weak of a mainspring. Your luger is cycling so fast that the magazine does not have sufficient time to get the next round in place before the bolt over-rides it, causing the jams you are seeing. I too tried good mags with very strong springs (Interarms Mauser mags), but that did not solve the problem.
There are two solutions for you. The first is to switch to a lower powered ammo. I have some old S&B ammo that fires a 115 grain bullet at a chronographed 1000 fps, and it functioned perfectly in my gun. The lower power ammo caused the action to cycle slower, allowing the magazine time to push the next round up in time for the bolt to pick it up.
The other solution is the one that I chose to allow me to use my standard 9mm P handload, which was to replace the mainspring with a new Wolfe mainspring. The new mainspring is noticeably stronger than the original mainspring. Before replacing the mainspring, my Finnish would jam as you described with Winchester white box, and most other standard ammo. After replacing the mainspring it functions fine with the same ammo. Apparently, the stronger mainspring slows down the functioning cycle enough to allow the next cartridge to get into the correct position before the bolt closes on it...
These are just my experiences, but they may be a help for you.
JD
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