the vibration from the power plant. The best results I've obtained are by resting the gun on a sandbag right in front of the trigger guard, this seems to be consistent with most springers, a rolled up cloth/towel is good too.
I think a gun vice would be fine because the gun can't jump like it can if rested on a solid object or bipod, harmonics should be consistent.
A good tune to reduce the harsh firing cycle is the most benficial thing you can do to a Gamo, the spring/guide clearance is considerable and creates a lot of vibration affecting accuracy.
