I built a backstop to have close access to for load developement and chrono work for 9mm and 300 BLK sub loads. The only other round shot would be 22s by my grandchildren. The timbers are 6"x 11-1/2" cresote bridge timbers and the center gap is 12". I can not decide what to put in the center, sand, packed dirt or maybe stone/gravel. I would appreciate your thoughts or suggestions. Thanks