Throwable spears/javelins must be made as missle-weapon pole-arms. To allow for a safe, light weight, their cores have to be hollow PVC only. Use 3/4" (schd. 40) with a 1/2" or smaller piece inserted inside. They must have a special throwing-tip. This consists of 2" of high impact foam over it. Throwing tips must be between 8" and 10" long and cannot be larger than a 3.5" diameter.