The tip of the harpoon is 3/4" round steel rod. I first drilled a hole to accomodate a pivot for the barbed point.(fig 3) Then I cut the rod lengthwise using an angle grinder and thin abrasive blade.(fig4) The same blade cut the point on the rod, and cut the barb free.(fig5) Finishing touches were put on the point with a bench grinder. A concrete nail is the pivot for the barb, it is welded in place. (fig2)
The back end of the harpoon is a 3/8" rod, The end was bent into an eye, wrapping it around a piece of rebar. A propane torch was used to soften the steel, and it was wrapped around a piece of rebar clamped against it in a vice. (fig 6, 7, 8)
The 3/4 diameter tip was then welded to the 3/8" rod, using an arc welder and 6013 welding rod. (fig 9) Yes, I know, the welding rod in the pic has a different #, but it didn't work as well as 6013. My welder works well with it. Four grappling hook rods were also welded to the tip.(fig 10) These also center the harpoon tip in the 2.5 inch diameter gunbarrel.
550 cord, which is rated at 550 pounds dead load was strung through 2 holes in a tennis ball, then wrapped 3 times around the eye, and tied with 3 half hitches. (fig 11) The tennis ball was then forced over the eye.(fig 12) The tennis ball is a snug fit in my airgun, and will push the harpoon down the barrel. It should be noted that a typical safety rating is 4 times the expected load. 550 cord falls short of this safety margin for anyone weighing more than 137 lb. But James Bond lives twice.
A steel rod bent into a "U", and secured to the gun platform with it's ends parallel to the barrel.(fig 13) This provides a support for the line which the harpoon tows. The line is wrapped around these parrallel rod ends in a "figure 8" pattern.(fig 14) This allows line to rapidly spool out when the harpoon is fired.
Directions for assembling the gun can be found in my instructable "Monkey Hunter Physics"
It should be noted that this device requires great care when launching. I have not yet launched it, and plan to launch it remotely in a safe location when I do. I have however used the airgun to launch a water filled tennis ball which towed 550 cord over a baseball backstop. This is also in the video for "Monkey Hunter Physics"