Loaded some 8x51 rimless for a MS rifle some years ago using 308 Win cases. Just ran them through the FL die and loaded up with .318 projectiles. Worked up a good load that was accurate and would have been virtually the same performance as a 308 Win. The owner of the rifle had got some dies made but had no cases. Just had to be careful to mark the packets with a warning and colour the primers to distinguish the ammunition from 308 Win.

I don't know where you would start with a common rimmed case to make them for your rimmed 8x51R.