As for ammunition, don't even think of giving up on a certain box after only 10-20 shots. At that number you've just begun to condition the bore to the lube on them. I got this advice from a long time .22 match shooter. If a new rifle that didn't shoot to expectation with his old match ammo, he would buy several box's each of proven quality ammo, shoot a box from a clean bore often at a single target, then shoot for score with the second box like a regulation match and see how it did, choosing the best of what he had to buy cases of. I heard him stress many times the importance of the lube/barrel marriage and he would not clean the bore until the gun was put away after the season was over.
I haven't tried this myself, however have you tried one of Paco Kelly's rimfire tools? It's a die where you drop a .22 cartridge in and use a rod to tap or press against the bullet, expanding it slightly. The theory is, most .22 rifles have sloppy chambers, (purposely done at the factories) to accommodate all brands and types of ammunition and the cure is to ''fit'' the bullet to the chamber.
Here's a link
www.gunblast.com/Paco2.htm