After some weeks of not being used pneumatic speargun pistons often refuse to budge at first when you try to muzzle load your gun. With the piston failing to move the spear tail is now firmly stuck in the piston which can only be jerked out with some difficulty. A way around this problem is to push the piston back with the gadget shown here before you next use your pneumatic speargun. They were made by Nemrod and each new pneumatic speargun was originally supplied with them in the sixties and early seventies. The knob on the top of the gadget made pushing down with the palm of your hand easier and if all else failed then you could invert the gun and push it down onto the gadget while pressed against the floor. Ideally a few drips of oil placed into the muzzle opening before commencing pushing is best as when the piston first moves oil runs into the inner barrel surrounding the piston nose which adds to lubrication of the piston and the front section of the inner barrel. In early Nemrod pneumatic guns the muzzle relief port holes were in the wrong place, they were at the front of the muzzle instead of the rear, so this gadget was essential to preventing the pistons corroding and seizing up.
Note that the gadget's rounded nose prevents it from jamming in the piston face, so you don't have the problem of a spear tail being stuck in the gun.