On page 47 the rule book reads...... " On a shoot through of a non-threat target that also strikes a threat target, the contestant will get the penalty for the non-threat target hit AND will get credit for the scored hit on the threat target. The reverse also applies when a round on a threat target penetrates a non-threat behind it. Hence the rule of thumb: all shot throughs count."

This would lead me to believe that it is OK to have a non-threat behind a shoot target that may get hit by poor aiming although a piece of steel would not give you a shoot through. It doesn't say anywhere that it is prohibited. Depending on the size of the popper, you could argue that it would be a procedural trap and thus poor course design.

Perry