A couple more hints....
As an addendum to the shot cycle physics noted above, don't oil the bore. Oiling will allow the pellet to move forward prematurely and not stop/slow the piston before hitting the end of the cylinder.
Might consider running a .22 patch tied in the middle of some 80# dacron fishing line or similar back & forth through the bore with a dab of JB Bore Paste to clean the lands/grooves and old pellet wax build-up. Clean out the resultant goo with a few patches moistened with isopropyl alcohol until they emerge clean and she squeaks her approval. Should tighten up your groups if old pellet wax had gummed up the bore's grooves....
Use some of James' moly lube on the barrel pivot and snug up the screws so the barrel doesn't droop under its own weight after being cocked but moves smoothly with your help. That will help keep the barrel from ever so slightly pivoting downward when the air pressure pushes against the pellet/breach. If you're getting any vertical stringing of your groups, might be the cause.
