Crossbow Pistol Plans, 1963

From the March issue of Science & Mechanics. The hard part is finding the coiled hood spring of a 1953 Ford to turn into the prod! Lacking that you could adapt this design to a replacement prod, or make a prod from strips of spring steel say .062 thick by 1/2" wide and stacking several different lengths to get the draw weight you want.

http://gallery.elsewhere.org/v/ephemera/Crossbow/