Using an airsoft 40mm grenade shell for the reservoir and valve system, build a nerf launching LAW for about 60 dollars plus the cost of one airsoft 40 mm grenade shell.
WATCH THE VIDEO!!! - It explains how the launcher works.
Edit: People keep asking about the Strap/Sling. it is a 1 inch wide cotton webbing strap purchased at a local fabric store. Buy about 1-2 yards. Also buy velcro and 1 inch plastic buckles.
Edit 2: People STILL keep asking about the "Locking Mechanism". Seriously, it is nothing but a piece of plastic. But since I get questions on youtube, here, forums, private messages and email, I decided to make a video. WATCH the video in STEP 7.
EDIT #3: If others have a method to use a DIFFERENT GRENADE, Please Post a comment. Then we can Collaborate on making an instructable for using a DIFFERENT GRENADE. I have brainstormed methods of using other grenades but they all BREAK my Rule OF K.I.S.S. With that said, I ONLY RECOMMEND THE MAD BULL XMPB4 or the S-THUNDER GRENADE. Don't Ask me about other grenades.