Well, you will have to recover the bullet after each shot. So I think I would build an open top box with plywood. Something like a foot wide, a foot high, and two feet long, and have a hole about six inches in diameter in the "target" end. Then you could put a square of cardboard on the inside to cover the hole before you fill the box with sand. After each series of shots, slide a new piece of cardbord between the shot up one and the wood, and pull out the shot up one. That way each series of shots will be pretty much equal so far as what the bullet is hitting. To make each shot exactly the same, you would have to replace the cardboard each time or not hit a previous bullet hole in the cardboard. (Aren't kid projects fun? ) How old is your son?