I got the idea from the halfbakery and i decided to "bake" it

http://www.halfbakery.com/idea/Pickle_20Harpoon

Step 1: Gather Supplies

For this Project you will need these things;

Small Spring Cannon (my other instructable)
duct tape
jigsaw blade
small thread spool
1/2 in wooden dowel
nail
2 washers
hot glue
scrap wood

tools;

saw
metal grinder
vice (or pliers)
hot glue gun
drill

Step 2: Make the Harpoon Gun

cut a piece of scrap wood about 2 in by 8 in by 1/2 in

drill a hole in the upper corner right or left (slightly smaller than the nail)

sandwich the thread spool between two washers and stick the nail through

hammer the nail into the hole so the spool can't move side to side, but can rotate freely

duct tape the spring cannon to the wood

Step 3: Make the Harpoon P1

cut the dowel to about 2 1/2 in

cut a notch at the diameter

drill a hole in the middle

thread the string through and tie it

Step 4: Make the Harpoon P2

use the vice to break the jigsaw blade in half
(clamp it and bend it until it breaks)

use the grinder to sharpen the tips

put hot glue on the notched end of the dowel

stick the blades in the notch throught the molten hot glue (teeth pointing out)

let the glue cool

Step 5: Finish It

make sure the harpoon fits in the cannon
if it slides out when pointed down use pliers to bend the barrel slightly
the harpoon should easily come out but not easy enough for it to fall out

the harpoon blade looks awesome