If we could all get our practical and theoretical heads together and do a few hours of brainstorming I think that a combination of both plus a bit of the bleedin' obvious would/could solve this problem quite easily. I wouldn't be starting with a blank sheet of paper but a few 'knowns' such as a) a gun that we KNOW will reliably shoot and function with 762x54R (yours'll do C-130!) and a magazine that we know will feed and function with a similar round. Yep, a standard and plentiful .303 Bren mag.

Together with a small workshop and................

Then start from there and deviate to 'unknowns' or variables if we need to. You game TacAdv? C-130? Joe? Brian?