Don't roll lap on 22 rimfire ammo. Just smear it in the grooves then wipe all you can off by wiping lengthwise, from brass to point of bullet. You can load them in the magizine. I shoot 5 shot groups till accuracy quites improving. Best ammo is CCI standard velocity long rifle, or CCI long rifle pistol match, because these are both the slowest long rifle rounds I know of. CB longs or regular shorts, of any brand, are OK if you are having trouble finding ammo. I just remembered the relitively new 60 grain ammo. This would be perfect if you can lay hands on it.
Use the same ammo for 22 magnum, but don't expect accuracy, nor use shorts. The cases will bulge a lot and possibly split, but pressure and velocity is extremely low because powder blows around the bullet in the long large chamber. No matter, they do a good job of lapping, safely, and there has always been enough speed to get the bullets out of the barrel.
I could tell you many success stories about lapping rimfire rifles, auto pistols and revolvers. All good, always a worthwile improvement in accuracy, and most times a DRAMATIC improvement, I've never a bad report. Here's a middle of the road one. My colt huntsman shot 1 1/4 inch groups at 25 yards with any ammo I ever tried in it. An old gun with many thousands of rounds through it, but shooting doesn't smooth a 22 RF. After lapping, any high velocity ammo would print 10 shots on a 25 yard steel plate without increasing the splat size after the first shot. This with a clamped on test scope and bench rest.
Maybe you need to hear another story. A customer who had three expensive target rifles, two shot into a half inch at 100 yards, consistently, the other shot quite a bit tighter. He lapped all three and cut groups in half on every one of them. Ahuh. 1/4 and 1/8 inch at 100 yards. I have no reason to doubt him, because if it would have hurt accuracy on those $1200 rifles, he would NOT have told be they shot better!
22 rimfires are the most rewarding guns to lap because it is so easy and ALWAYS improoves accuracy.
