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UPDATE: This thing has a very short range. If you build this, you would be lucky to get 150 feet. I have since taken it apart and rebuilt it with a 24 inch barrel.
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Cost to make - roughly $15 plus cost of Mad Bull XM PB4 unit.

My cost to make - $3. I had almost everything I needed already.

HOW IT WORKS:
The Mad Bull XM PB4 is a Rerservoir and Valve system for pressurized volatile liquid. For airsoft, this liquid is called Green Gas. When the Valve of the PB4 is triggered (opened) all the liquid flows out of the reservoir into the Chamber behind the Nerf Football. The liquid - being very volatile - QUICKLY vaporizes into a gas state. The volume of the Gas is hundreds of times Larger than the volume of the liquid state. This Rapid Expansion from liquid to gas is what Pushes the Nerf Football out of the launcher. For further explanation, please talk to your local high school science teacher.

UPDATE: I took this launcher apart and built a longer launcher. The new launcher is 24 inches long. It is a compromise between the pistol and the LAW which extends to 30 inches long. It abandoned this design because the range sucked.

Step 1: Tools and Materials

Tools

Dremel / Rotary tool with the following attachments
- Cut off wheel
- sanding drum
- shaper / cutters
Drill with assorted drill bit
Screw tap
scroll saw
hack saw
screw driver
Clamp (optional)
Sandpaper

Materials
ABS or PVC Pipe 2" schedule 40
Shielded pipe coupler (see my M72 Law instructable).
Wood
screws
aluminum bar
steel rod

Mad Bull XM PB4 airsoft grenade (also sold by RAP4).

Step 2: Make a Pistol Grip.

Get your piece of wood, your saw and your dremel. Carve out a pistol grip.

If you do not know how to do this, find an instructable on how to carve wood and how to use a dremel.

Step 3: Cut a Channel on the Rear Portion of the Pistol Grip.

A push rod needs to link the trigger to the lever that pushes the valve on the PB4 grenade. A channel needs to be cut into the wood for this push rod. Use what ever Dremel bit you have to cut out this channel. I used my cut off wheel.

Step 4: Trigger Well

Make a channel or well for the trigger to go into. Same method as the channel for the push rod.

Step 5: Connect the Push Rod Channel to the Trigger Well.

Use your drill to connect the push rod channel to the trigger well. Make the hole wide so the trigger can travel.

Step 6: Make a Trigger

Make a trigger out of you aluminum bar.

Picture 1 - Measure and draw out a rough trigger shape. The two dots are: Upper Dot = hole for the push rod; lower dot hole for trigger pivot point.

Picture 2 - Test for clearance.

Picture 3 - shape the trigger

Picture 4 - Test trigger travel, fit, and clearance

Not pictured - drill out the dots.

Step 7: Pistol Grip, Trigger, Push Rod Assembly

You may need to make a shim so the trigger does not move side to side or wobble. I used a piece of brass (picture 1).

Picture 2 - Trigger is now in place. Notice the brass shim.

picture 3 and 4 - test the push rod travel

Step 8: Activation Lever for the PB4 Grenade

I purchased a small hinge for the activation lever. The lever is just a flat aluminum bar with a screw sticking out of it to depress the grenade valve.

Step 9: Pistol Grip and Grenade/coupler Assembly

Connect the coupler containing the Airsoft grenade to a section of PVC/ABS pipe. Attach the pistol grip assembly to the pipe using a cobination of epoxy, bondo, hose clamps, zip ties, etc. What ever makes the pistol grip secure to the pipe. If using screws, make sure the screws are flush with the inside surface of the tube so the nerf does not get snagged on it. When using glues and epoxies, make sure the pushrod does not get any glue on it. Connect the push rod to the valve activation lever. All done!!!!

Fill with green gas, load your nerf vortex and play SAFE! Enjoy.

Performance
Outdoor test showed about 120 feet effective range due to much shorter barrel. Max range is roughly 150 feet.

NOTE:
Those familiar with Airsoft know that "Green Gas" is really propane and is really flammable. With that said DO NOT attempt to use a projectile that is on fire.

I DO NOT CONDONE USING PROJECTILES THAT ARE ON FIRE, E.G. FIREWORKS WITH LIT FUSES IN THIS LAUNCHER. DOING SO COULD RESULT IN PROPERTY DAMAGE, ARREST, INJURY, AND/OR DEATH. USE ONLY SAFE, SOFT PROJECTILES ACCEPTED BY THE AIRSOFT/PAINTBALL COMMUNITY.