Case life for the .256WM is short in the Marlin and in the TC Contender.
I made some rimless cases from .223 Remington bras and this helped some. The cases are different (reduced) capacity and change your load data. I made up some long neck wildcats based on the .223 case and they had promise but the .25-222 would be an easier case to make and work with.
I enjoy small game cartridges and rifles but the .256WM is a lot of work. Bob Millek back in 1993 wrote an article: Why the .256 Winchester Magnum?
His case life was not any better than mine


256WM Broken case
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256WM Rimless
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Slim