There are several 'schuetzen' cartridges based on the 357 Mag or Maxi case necked to '32', but it tends to be the 32 as in 32-40 bullets, not the smaller dia. pistol bullet. One necked to take the readily available .308 bullets would be good too.
As regards 25 cal., Id suggest the 25-35 also. I had a custom stubbed barrel from a member here, 1:10 twist and it will stabilize the heavier 117ish and shot the 60, 75 and 87s as well as the slower twist 25-20 I once sorted out. Most of my experiments were with cast bullets from a schuetzen context. The 25-20 just couldnt stabilize the much more ballistically efficient +100gr. bullets (87gr. was about it)and proved the theory that more twist was better for slower lead bullets from sub-sonic on up. Limited jacketed bullet experiments proved it was a shooter, but the 117s were not what Id call fast and 100s were my varmint choice for top weight. These considerations will be the same with any limited powder capacity case as in the 256.
Also, mine was more of a bull barrel, and with that tiny hole it was stiff, and stout. These calibers would be nice in the superlight taper.
I concluded my work and sold it here to a member as I really am not a varminter or schuetzen shooter anymore and it was doing too much time sitting. My criteria for custom calibers are cheap, readily available brass, no complicated/troublesome case forming, std. dies and good bullet weight selection (especially cast bullet, non-custom molds) if you want to get the most out of your choice and shoot more than mess about.