I need information on the 256 GNR

Questions include:
1: Performance in various barrel lengths/ bullet weights.
2: Compatibility with 256 Win forming dies (I've got a friend who has a set to sell, hence the question ) to reduce the Max brass to .257, or does it come with forming die or dies?

Specifically, I'd be interested in a Contender barrel, between 12-16 inches. I suspect a 13-14" will end up getting the nod for convenience.

Looking for an in between cartridge that would be good for vermin but also work on deer sized game at reasonable hunting distances.

I've tossed this around for some time and I really like the idea of the standard 256 Win mag, but may be a bit lacking in the deer category as the Max based version would have a ballistic edge or a bit heavier bullets.
I've also considered the 6.5 Grendel as it is factory, and I'm a bit of a 6.5 fan. However, being rimless, it just doesn't seem traditional for the Contender. Yes I know that the Contender has always been chambered in rimless cartridges. So the 257 or 6.5 Raptor would be a good choice too.
Lastly would be the 7x30 Waters or 7 GNR, either would fill the bill but be a bit on the bigger side. In my mind would be a bit more of a deer cartridge and not so much for smaller vermin.

Forgive the mental gymnastics I'm going through, That's why I like the idea of the Max based 25. I think it would be equal to a 25-35 Win in velocity, but much more efficient with powder. Light bullets down to 70, and heavies to 120.

So, Gary and fellow misfits. I'd appreciate your data and your preferences for this type of 'niche' cartridge. Like I said a "tweener" cartridge that is easy to shoot and is cheap to feed in powder.

Any information would be appreciated.
Thanks in advance

A.S.