Here is how you can make your own water balloon launcher. We use high strength surgical tubing unlike many of the launchers you can buy online that don't shoot very far. The cost of materials is about $18. This instructable covers only the launcher. See the Water Balloon Launch Pad for instructions for the frame.
Step 1: Materials
(1) small roll of duct tape
(2) 8 inch lengths of 1/2" vinyl tubing
(2) 6 foot lengths of latex rubber surgical tubing
The vinyl tubing can be purchased from a hardware store. The latex rubber tubing can be purchased online. One good source is McMaster part number 5234K75. It should be 3/32" wall thickness (not 1/16" thick). The surgical tubing sold at hardware stores is normally only 1/16" thick and not strong enough to launch a balloon 300 yards. If you are in a pinch and don't have time to order the thicker tubing, then you can double-up the 1/16" tubing, but that gets quite expensive.
Step 2: Pouch Layout
Attach together 5 pieces of duct about 18 inches long. Fold over the top and bottom edges about 1/2" to create a hem. Take 2 pieces of tape about 7 inches long and fold over one edge about 1/2" to create a hem. Place these 2 pieces with the hems facing each other about 3 inches apart across the 5 pieces of tape.
Step 3: Add Tubing Channels
Place the two 8" pieces of tubing across the 5 pieces of tape about 8 inches apart. (The threaded rod in the picture is only used to keep the tubing straight during this step). Fold the ends of the 5 pieces of tape over the vinyl tubing and adhere them back on themselves.
Step 4: Handle
Make a handle by attaching 2 pieces of tape about 22" long lengthwise. Then fold or roll them lengthwise into a 1/2" wide strip.
Step 5: Insert Handle
Cut two 1/2" slits 3 inches apart in the middle of the pouch. Do not cut through the hemmed tape embedded in the pouch, rather cut right next to it. Insert the handle through these slits. Leave a loop of the handle sticking up big enough to easily fit your hand into.
Step 6: Secure Handle
Wrap the ends of the handle around the pouch, overlapping the loop of handle that was created in the previous step. Secure the overlapped portions of the handle together by wrapping them with tape.
Step 7: Attach Surgical Tubing
Insert the surgical tubing through the vinyl tubing channels and tie each one together with a square knot.
Step 8: Bomb's Away!
This is a 3-man launcher but we've attached it to the Water Balloon Launch Pad which only requires one person to load and launch. Have fun!