The rule of thumb, is .005" to .010" smaller than the bore. With that .22 bl. I would think the larger the ball, the better as it will
swage into a slightly elongated projectile. I did this with a shallow grooved Sharon .36 barrel. I used .375" balls and normal denim,
about .020" thick. A very smooth radius on the muzzle's crown allowed the ball to form/conform into the bore. It shot quite well.
A ball punched into the muzzle on a strip of cloth using the starter, then pulled out with the "tails", showed a slightly flat ended,
elongated slug with a perfectly round base.