I heard something once about using a Hilti blank and an appropriate sized shotgun pellet for those .25 rimfire rifles.
Seeing as a .25 is bigger than a .22, the gunsmith would probably have to sleeve it at great cost. He could probably throw a new barrel on, but the cost would be more than the guns worth.
The Hilti blanks can be bought at any hardware store, they are used for nailers. Little bigger than .22 blanks. I would suppose the pellet is maybe lightly glued to the front of the blank.
You won't be winning awards for accuracy, but it will at least allow you to hit pop cans a couple feet away.
PM Dingus, I think he is the resident antique rimfire specialist.