Don;t take this as an insult - but are you aware of 'Mauser's RUle' and how the case determines the geometry of the magazine box, etc? The cartridge is from where all blessings flow when it comes to Mausers. If the rounds are not held securely, something is out. Could it be the spring ? sure. Test that by taking it out and standing it up right then gently push the follower down - does it dip/collapse at the front or rear at any point in its travel? or does it maintain its integrity throughout?
If you have three rifles that do the same thing - it looks more systemic. I have gone through a similar issue with a Mauser 9.3 and we found the issue by starting at the cartridge and checking the required magazine, etc dimensions from there. Once those were corrected, good to go. See mag dims below.
It;s quite possible your issue is not solvable with a simple fix that doesn't alter the rifle and you might have to live with it. But that doesn;t mean Rigby made a trash or unreliable rifle - it is what it is. And yes, there is a bit of tolerance with the dims where a rifle will work 'fine' when not built exactly as intended. But the exception doesn't disprove a rule. As a rather decent builder once told me, "I've never seen a Mauser that was built as intended that did not work as intended."
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