Here is the gun stock:

There is an annoying tab of plastic protruding from the grip which hurts your hand. The first order of business was to get rid of it:


While opening the gun up, you need to take off the yellow and red rings of plastic. I recommend using a Dremel.
When you open up the gun, a spring and a ring of plastic should pop out at you. These aren't necessary. You will notice that it has a mechanism which doesn't allow the trigger to be pulled if the lever isn't completely down. I would leave this in place, but you can take it off. Here are the internals:

Take the barrel and plunger tube out, and throw away the barrel. Take 5 inches of a tight fitting barrel material, I used CPVC. And glue it directly onto the hole. Glue it well, or the barrel will shoot out. Sideways!!

Close everything back up and test it out.
My ranges were about 80' - 90'. I'm sure someone could do better with a spring replacement, but I didn't feel like it.
This is a great gun for the little amount of work I had to do. The leverage makes it incredibly easy to prime.