Put a barrel tuner on it, or thread the end of your barrel and put a heavy muzzle brake on it.
You could also press a fluted steel sleeve pressed over the barrel to stiffen it up.
If you want something more accurate, a bolt-action rifle is the way to go because the rifle won't be affected by the bolt slamming back and forth when it cycles the action.
IMHO, trying to get a lot of accuracy out of the 15-22 is a waste of time and money. It is what it is.
*Aim small, miss small...