Late to the party ... I don't get here very often ... but Bertram brass in Australia produces brass for a shortened 50cal. Case length is 2.1", case rim size is 416Rigby, case head is either 500Jeff or 505Gibbs dimensions (need to check), 20deg shoulder. He's used it as a subsonic 50cal project with a 750gr projectile. Might be worth inquiring with them. I'm exploring its potential for pushing a 535gr at 2150fps from a Ruger S/A as a compact big bore rifle.
Cheers...
Con