Hang a pulley up about six feet and tie a length of string between a gallon jug (full of water) and a 4lb weight. The gallon jug will hold the weight suspended in the air. Shoot at the jug from 80 to 100 yds off hand. The idea is to have the weight hit the ground within 25 seconds of the first shot. A shot in the middle will usually take too much time for the water to flow out. A shot near the bottom will release enough water to drop the weight, a hole near the top won't. The reason for the time, is to allow a reload, if they can do it fast enough and still hit the jug. The group I saw do this, tied lead weight to the string which would fall on an oversize trip pan of a trap which had a flag attached to it's jaws, so when the trap sprang, the flag went up..