Madsen was unique in that most of their mags were proprietary.
The magazine for the gun was dependent on the calibre. There are some exceptions to this but not many.
The position of a stud on the front of the magazine prevented them from being used in other calibres however the mags could be modified if need be.
The Madsen was made in .303 British for a test trial in Englandicon but was never adapted.
Somewhere I have the British manual on the Madsen made for the UK and most of the equipment for it was adapted for cavalry use.
The British made a direct copy of the Madsen in .303 called the REXER.
They are very far and few between.