Try Our handy checklist.
Groupings checklist in order.
Remove your silencer if you have one and check groupings. If groups are much better your silencer will be faulty or not fitted properly.
Assuming you have removed silencer/mod.Make sure you set your parallax at the distance you are grouping at (eliminates parallax error) Check you are using the same pellets as you normally use to get decent groups.
Tighten all scope mounts, check groups. if still not correct move on.
Clean barrel and check barrel crown for dirt. Fire 30-50 pellets through newly cleaned barrel,check groups. if still not correct move on.
Load a pellet and watch carefully as your pellet probe pushes the pellet in. If the probe is off centre it may be damaging the pellet.If it looks ok try hand loading a pellet by pushing it into the barrel with your fingers. Hand load five times and check groups.
Okay we are now down to three possibilities, regulator problem but this would only effect pellet height. if groups are still out to left and right you can discount regulator (if you have one).O rings and breech seals. (it's almost always the breech seal ) They are super easy to replace.
Your scopes F*****? Borrow a scope you know is sound and try it. If your groups are good it is probably your own scope that is faulty. Check it..
To check your scope stick it on a borrowed gun and try to zero it. If it doesn't zero and you get bad groups on a good gun you will know it's your scope. To test your scope on a known good airgun do the following.Fire one shot at your target. turn your elevation turret UP by 20 clicks and fire a shot at your first pellet strike (you should shoot higher) Turn your windage turret 20 clicks right and fire a shot at your second pellet strike (you should hit right), turn your elevation turret DOWN by twenty clicks, fire at your third pellet strike (you should fire lower) Turn your windage turret LEFT by 20 clicks and fire at your fourth pellet strike and this time because you are back to where you started you should hit your very first pellet strike. If you don't then your scope is broken.
If your scope is not broken change the o rings on your breech seal and your magazine.If you are still getting bad groups get a small hacksaw and cut ten inches of your barrel and five inches of your stock. That way your gun will fit into the bin much easier.