That was more or less my idea as well, with the .25-20. Then ran into a brass availability problem. And also discovered that really small cast bullets can be a little fussy to get to shoot well.
Others have made the point the .25-20 is considerably more effective on game that the .22, and I suspect that would be true even with subsonic loads.
The Hornet is popular, but can be a little fussy to get to shoot. Brass has decent availability. The Fireball is very unfussy, and can be made from .222 or .223 brass. The Bee is unfussy, but brass is not available at the moment, except at scalper prices.
I think there is a wildcat named the .22 Flea, which could be described as a shortened .22 K-Hornet, designed to use very small powder charges.
The project is interesting, and may prove a challenge.
