"sweet spot" where you have the least extreme spread from your first to last shot, many like a 20fps extreme spread other use some %.
There is a balance between striker weight, spring strength, valve stem return spring and psi in the airgun.
IF there is too much psi for the striker/spring to over come you will get VERY low fps. Due to a faulty gauge I once filled a big-bore to 260bar this resulted in 0 (zero) fps, didn't move the slug at all. We have an AA410 that likes 2,700psi this allows it to start shooting at 900fps, fill it to 2,900psi and due to partial valve lock the first ( and several thereafter ) are around 800fps until it gets down to 2,700psi then it shoots just right.
John