Tuning Gamo magnum .22 accuracy

If its only off for the first shot, it's possible that some lube is seeping down past the seal while sitting for a long time. First shot burns it all off but will through the first shot off course with the rest. Gamos do have a tendency to be over lubed from the factory. A proper lube job will fix this problem.

Agree totally...ifvthe scope was moving it wouldnt return to O after a half dozen shots. A simple tune with good and proper lubes will probably cure most of the problem. To test this idea, check each shot over a chrono and see if velocity changes as things warm up!
 
After the rifle has set say overnight, get a chronograph and keep track of first 25 or more shots. If velocity is fairly consistant then there are other issues to cause your stringing. If velocity is all over the place then you have a place to start looking. Leaking breech seal, piston seal, canted spring, excess lube, etc. Have to find a place to start looking. Normally the lube is gone after 500 shots, but did it do any harm b4 getting shot out??
 
I did some testing today with a Caldwell chrono. It’s touchy. Didn’t ready a lot. This is on a rest at 26 yards



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Sure looks like dieseling. Lube still seeping out when first couple shots taken.

Probably needs couple hundred more break in shots before it could naturally settle in.

Dont expect good accuracy until fully broken in when it no longer diesels.

I would remove the scope with mount and shoot open sights for now until it stops dieseling.

You will know when the shots stay consistent over the chrony.