Easiest, cheapest donor for a 500A2 is the CZ550 in 416Rigby. Bought one just for the conversion at a later date. The number of 50cal pills just keeps growing so I reckon its a better choice than the T-Rex ... can you get brass for the T-rex? I'd rather a Nyati than T-Rex ... at least there are a few sources for brass.
Cheers...
Con