I have a couple .25-20 molds which I use for feeding a .25-20 Single Shot (not .25-20WCF, not that it matters). All are in the 85 grain neighborhood and are plain base designs which means roughly 1400fps max, maybe 1600 on the outside if you're lucky (or carefully plot alloy hardness and throat/bore dimensions). I'll be doing a casting marathon in the very near future and would be happy to make an extra handful for you to try. (I gotta do some .22 bullets for Penndog at the same time so no big deal.) PM me and we can move forward, if you wish.
As for powders, there's a whole bunch suitable for .25-20 work. Even though my experience is with the Single Shot version, case capacity is nigh identical with the WCF version, and 2400, 4227, 4759, Unique, are all eminently usable in my experience although you can go overboard with Unique if you're not careful. My favorite is 4759 but I'm loathe to mention it since it's discontinued. Betcha a case full of 4198 would work too without maxing out pressures.
