OK, that tells us that the chamber in this rifle is long, relative to the ammo, even that with the greatest length base-to-drive band. You may therefor be at a dead end with this particular rifle.
Something that has helped me in the past is to bump the bullet diameter up using a Waltz die. For my long-ago testing I had built a homemade die that works like the Waltz. It shrank groups from the test rifle, a stock-barrel 10/22, by about 20%. Reason being that the "sporting" chamber needs a .225 bullet to fit, but the ammo was .223 or .224 out of the box. The control diameter of my tool was reamed to .2250. If you have a lathe, or access to one, it's not a difficult job, but you will have to buy a couple of reamers at $25 a whack. I can send you the necessary drawings.
Here are some old threads on .22 bullet bumping on the forum.
http://castboolits.gunloads.com/show...-22-Sizing-Die
http://castboolits.gunloads.com/show...35#post2745735