I am working on something similar to the.221 Askins, a reloadable 22LR.
Askins had a similar idea and used 5.56 Velo Dog brass, which is almost impossible to find now a days.
I am using .25ACP brass sized to a straight walled case with a rim that will hold a 22 cal bullet.
I am using 22 cal pellets with a primer to replicate 22 Colibri Ammo to start, then I will use a small amount of powder to get 22 short and maybe long rifle velocities.
The primer pocket and bullet occupy case capacity that the 22LR does not.
An adapter was made from a piece of 22LR barrel the dimensions of a.410 shell to fit a .410 bolt action I have.
Initial tests have been positive so the next step is to rebarrel a bolt action 22 and convert the bolt to center fire.