Here is what the tank looks like. It's an old XP150. I used to love this gun :).

Here is the unmodified gun, along with a 22 in barrel of 1/2" sched 40 PVC. The long barrel is helpful because it makes the darts more accurate and it gets a better range. The super soaker is good to use with a long barrel because it is designed to throw out air for a long time. The impulse time is longer than a normal Nerf gun. So a long barrel helps a lot. The piece of wood is a small dowel cut to a little longer than the barrel. It's used to ram the dart down into the barrel.

The first thing to do is cut the nozzle off. The nozzle regulates the air and doesn't release enough for it to make a good nerf gun. Cut it off and you will get the air you need. Save the pieces that come off when you cut the nozzle off; you will need them later.

You should also cut the thing on the top off. It's easier to tape it when the thing has been cut off.

OK, now take the outer piece of the nozzle assembly and slide it over the PVC. It should fit pretty well.

Tape it in place.

Take your barrel assembly and tape it on to the gun. Try to get the hole in the PVC to line up with the hole in the gun. Make sure it's airtight; use lots of duct tape! Now you can shoot it. Ramrod a stefan or micro down the barrel and pump it and shoot it. BUT the barrel is really flimsy. Now we're going to fix that.

Cut a piece of 1/2" sched 40 PVC to about 11 inches. Cut a piece of 1/2" CPVC to a little longer than a strip of duct tape.

Tape them together.

Now tape the PVC to the top of the gun, and the other end of the assembly to the barrel. This strengthens the barrel a WHOLE lot and it gives you a place to put your ramrod (in the top piece of PVC).

On the XP150, if you just wrap tape around the top of the gun, it will cover the slide for the pump. Just cover it over, then when you're done, cut the tape on the sides of the slide and pull it off. It still has a good enough seal, and it lets you pump it normally.

The finished gun with barrel strengthener.

The whole gun before I put on the barrel strengthener.

This gun is REALLY nice. I pumped it 10 times, not knowing how much power it had. I shot a hot glue tipped stefan at the wall, and it almost completely destroyed the dart. I wouldn't use it inside ;). Besides, the gun is HUGE. With a 22 inch barrel, it looks almost as long as the Ultimator. It's more accurate though :). This should work on any super soaker, but some might be better than others. The XP150 works great, and I'm pretty sure others in the XP line will work well too.