OK, so I now know for sure it's not the spring tension in the rotary mag. that's the problem. The tension & spring are just fine.

The jamming occurs right at the case lower shoulder - exactly on the edge of the shoulder/main cartridge case body.
As the bolt pushes the cartridge forward just to right there's a small 'lip' that should deflect the cartridge upward... it doesn't & so the case jams right at that point.

What to do about it???? - things that make you go 'Hmmmm'!
Maybe I'll try gently polishing it with a Dremmel - but before I do - I'll think some more.
ATB, deeangeo