Can you find, or do you have, 357 Magnum brass?

You can make rimless 256 from 223 cases.

When I first made some rimless, I simply removed the decapping spindle from the sizing die and ran the 223 case in. Then trimmed to length. I did not have the ream, but that may change depending on the cases used.

A bit hard to extract from a Contender unless you change the extractor, but shootable.

What are you using for bullets?