Can you fill the magazine with cartridges? Can you put two in, then push on top of them down into the magazine and do they return to the top against the feed rails?

Are they jamming in due to OAL?

Your problem also could be due to the fact that Mauser actions are designed to feed one caliber. Changing to another caliber can often cause difficulties ranging from mild to serious. SOME material might have to be removed from the front sides of the feed rails. If you know where to remove it it is easy, but if not, you can wreck an action pronto. Better get it to a good gunsmith who has experience rebarreling milsurps and making them behave.