I found this idea. The caveat is I have no idea if it works or is safe. Any further questions should be directed to Taxidummy at Aution Arms. Once again I am not endorsing this idea. I do not know of anyone currently producing .25 RF ammo.
"An idea that I have used for 25 RF shooting in single shots (Stevens etc) is to drop a 257 round ball in the chamber. Sometimes a little wooden "breach seater" stick is necessary to get it to the rifling. I have a friend that uses soft cast (30 to 1 etc) 25-20 bullets in place of the RB. Then fill the chamber with 3F black powder or a BLACK POWDER substitute like Pyrodex, (maybe Tripple 7 ?)to within about 1/4 inch from the top. One of those little pistol flasks or a Lee powder scoop works well for this. Last I use one of the Remington 25 cal nail gun charges (just like it comes from the box) for a "primer/chamber seal" and insert it into the chamber making sure that there is enough powder to get a slight "crush fit"! They work like a champ on Prarie Dogs etc. And admit it guys it's not that hard to clean a little powder fouling from a Stevens 44 etc. ABSOLUTELY NEVER NEVER!!! USE ANY KIND OF SMOKELESS POWDER IN THIS MANNER!!!!! By the way, #3 buckshot is .257 but I've used a lot of #4 also. With a bag of buckshot, A pound of 3F, A box of nail gun charges, and a measure you can have that old #44 up and running. And a lot of fun to boot! Some 25RF guns were pretty flimsy And many are worn and unsafe to shoot with ANYTHING! so use your own judgement on this and ALWAYS WEAR GOOD WRAP AROUND SHOOTING GLASSES!!!!!!!! "