Thankyou Kuduae for the explanations & hoping you didn't mind me posting the photos which I view as extremely informative. I think the variations of cartridge/cal. types of the shoulders in the rotors really fantastic and have always been impressed with the fine engineering associated with MSch rifles.

For a while I thought I had a problem with the rotor spring..I took it to pieces to check it out. Clearly that was a red herring and there's nothing wrong in there at all.

Then I believed the release tab was the culprit...another incorrect assessment.
Finally the magazine itself, but no faults could be found there other than the shoulder ramps appeared to be for a different 'sideramp' feed. Which Iowa303 confirmed.
So, my magazine comes from a different rifle with a different sideramp in the action.
Anyway, feed issue resolved now thanks to info from those on this forum.

But, - That .458 definitely looks 'chunky' and in a nice way.
Cheers, deeangeo