25 Hornet...
270 REN
I'm ok really with either cartridge, & like I said earlier, am currently building a Contender barrel in 270 REN... but was thinking a 25 caliber for the revolvers, & could shorten the case as much as necessary to allow fitment in the cylinder of the gun with the shorter cylinder ( likely the Single Six )... this is an improvement on doing up a revolver in 25acp ( except for factory ammo availability ) in that a 257 bore barrel is much easier to come by than a .251 barrel, & the longer case ( even if trimmed to 357 mag length ) offers more power / versatility than the short 25 acp case...