The standard rimmed round for Siamese Mausers is the .45/70. I put one together for a friend back in the 70's.

It gave the same ballistics with identical loads as my .458 2" M98 VZ24 Mauser.

2,300fps with 350gr., 2,160fps with 400's and 2,060fps with 500's.

Seems to me, with the 500's, it used the whole magazine - no room for a longer case and nothing of value gained by using a longer case with shorter bullets.