Hi, Paul.Have you checked your stock and scope mount screws? I'd recommend a more thorough barrel cleaning, or at least pulling dry patches through to get rid of any residue from the WD40 treatment. Then see if the chrony strings stabilize. It's not unheard of for a piston gun to settle in a little slower than it was out of the box. It may be because excess factory lubes are now burned off.If you are pretty sure the beech seal is leaking, you can try to carefully remove it and put material under it (dental floss is popular) to shim it up.The differences in POI may be because of changes in barrel harmonics related to the change in velocity. Good luck!
Rule of thumb for air guns, not clean the barrel until accuracy goes out the window. These are not like firearms where you have hot powder burning and melting &^^& to the barrel, the lead acts like the grease in a cast iron skillet....Just ask your Granny what happens when you clean her cast iron skillet with soap and water!!!!!
Could be rough bore. Try slugging it (push some tight-fitting pellets through) feel for rough/tight/loose spots, and burrs at the crown.Alot of guns benefit from some lapping/polishing. I regularly use a sharpened pencil and lapping paste in a cordless drill to polish the crowns, as well.Turned alot of clunkers into shooters with a little polishing.Also, piston seals often come damaged from the factory, this can affect consistency
Judging by me.., circumstances to get lost with a magnum jumper. Even more when you are considering some other possible cause(s), 'the shooter' may be not enough focused, and being the main cause of several errors.Btw, something that helps me is that an ES up to "4"% of MV does not cause a relevant POI fluctuation (to my needs). And, even if ES is more than 4%, ChairGun tells how much of POI vertical changing would be - due ES.