How to test if gun will Cock

To save you putting your spring gun together and find it will not cock then do the check I have shown here.
To make sure you do not have too much Spring Guide and or Weights then you can do a simple check before you put it together.
Drop the spring guide into the piston and make sure the rear of the guide touches the rear of the piston. As shown in the first photo.
If it stops short like the second photo then it probably will not cock.
Obviously this is not counting on the mainspring having too many coils which again would prevent the gun from cocking.
Some guns also need a little bit of over travel to trip the sear into locking the piston rod.