Hatsan Hatsen Flash QE .22 accuracy issues

You said it shot good for awhile until left alone with your son. Me thinks your son did something to it. Or the barrel needs to be scrubbed well.
While anything is possible I do trust him. He's careful with all our guns. My daddy taught me well and I'm carrying on the tradition.

@BABz58 that looks a tad rough there.
 
Here is a good barrel prep tutorial.
 
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Coming back here to answer this issue.

After a bunch of testing, I found exactly one pellet that shoots great. The H&N Baracuda Match 21.14 grain. I wouldn't call it fantastic accuracy but black birds and rodents are dead out to 35-40 yards easily. No changes made to the gun, just tried a bunch of different pellets. Any other pellet I've tried shoots like a scatter gun.