Hatsan AT 44

Check if your air tube a stock screws are tight. I was getting the same results until I chrony and adjusted the hammer spring. My best shoot didn't start until after 180 bar. 

"Dereck"I have a Hatsan AT 44 in .177. I set the rifle scope and if I leave it for a few days, shoots completely off from the settings. Also if I fill the tank after setting the scope. I have used several scopes and still have the problem. I fill to 200 bar.

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What do you mean by "completely off from the settings"? If you mean it isn't holding a zero, can you describe the conditions you are testing it in and what you were expecting it to achieve? 

Is this a new problem or has the gun had this problem since you got it?

If it is a new problem and you are certain that there is no issue with your scopes, it could be the rings or rail (if you are using a picatinny riser over the dovetail). If you are switching scopes without removing the rings, try remounting the rings.

Also try cleaning the barrel. A build up of filth in the barrel could easily cause erratic shooting.

If the gun is new and you were expecting it to hit the same hole at 50 yards every time, that's not going to happen. How far from your zero is the poi?

You'll know if the air cylinder is not screwed in properly with those guns because it will make a snap / pop sound coming from the gap in the threads when you shoot. If it's not the QE model then you might not hear it over the sound of the shot but making sure it is tight is easy to check. 

If it is a QE model then unscrew the end cap and try and look for any clipping inside. Obviously check it's not loaded before doing that...

Also check the stock screws, if you removed them, it is easy to put them back too tight. You'll know if you unscrew the air cylinder, take it out and put it back in. If the screws are too tight then it won't fit back in as easily as it did before. The safety switch might feel tighter too.