My gun is very weak - some darts fail to leave the barrel altogether, other travel for about 50cm. Is there a way of increasing the pressure of the air pump or something so that it will work properly again?
I just picked up this game in a thrift store (brand new never used copy) and have exactly the same problem (which is presumably why they ditched it). Mine would not fire at all unless you put the dart in only three quarters of the way in which case it fell out the end
I have got it to work now.
Here's the trick.
Pull the gun apart with a small phillips head (cross) screwdriver. You need to pull the mechanism apart (take note of which piece goes where and where the rubber bands are.
When you take the black barrel piece off the gun casing it separates into two pieces.
The problem with the gun is the pressure seal between the stopper and the barrel. On my one there was some residue of vaseline or petroleum jelly which gave me the idea of packing some more in there. (You can get it from your supermarket in the shampoo aisle - also great for bad chafing).
I smothered the stopper bit and put it all back together. The gun works now - although it shoots a little irregularly - but every shot goes across the room.
My next port of call was to pick up a grommet from a rubber supplier the size of the barrel and glue it to the end of the stopper forming a tighter seal.
Last resort - buy a cheap nerf gun from your local toy store. They're pretty cheap (at least here in Australia they are).