Quote Originally Posted by Shed tuner View Post
if it's not the length of the guides it may well be the centralisation of the rod in the piston - is it centred correctly ? Also is the rear guide "brim" OD the full size of the main tube of the gun, thereby preventing it from moving off centre ? Also look at the trigger block - does the lead into the hole in the trigger block through which the piston rod passes show any signs of being hit / metal contact ?
Would have to be a mile out to prevent cocking

also check any "power washers"on the rear guide are of the correct diameter to go inside the bore of the piston .