Fun Topic... One I spent much time on.
First built a 8ft tall, 10ft wide, 2 feet deep box out of 1 1/2" square tube. add roof pitch to keep keep rain out of the future sandbox. Do this in your shop or something and put the first course of wood or 2 on the bottom. Suspend 3/4" thick "stall mats" from a hanger of sorts I think I used perforated angle (like garage door hangers)...
WHY? - the rubber mats will hang behind the final wood surface and be filled with sand. If you miss your targets too many times, and have to replace a board, the sand wont all just spill out!!! I've heard of mats for "self healing" but not for sand retention when facing is taken off. (this is my original design about 5 years ago. Though I see some variations on it online today..)
Haul it out using your front end loader and a chain!
Finish wrapping the frame with wood, I used 2x12 PT wood and finished with "
READYSEAL" I found this product some time ago and love it.. Sold in many Home Depots and Lowes in Texas as it is a texas based company, but great stuff... it may drip or run if you slop it on, but it never ruins the project.. you can slap it up as ugly as you want and somehow it dries consistently.. Plus and recoats years later require no work except a quick pressure wash and dry. sorry, I digress.....
Then fill with a lot of dry sand. Attach some OSB for a roof, and finish the roof. (i used plastic corrugated roof from Home Depot). Wet sand will allow serious bullet over-penetration, however I think a 50BMG will stop in less than 18" of dry sand if I'm not mistaken. Wet sand = no good!!


I did add some "support" on the back side and boarded that in as well, I keep my compost/dirt pile behind it. I did "wallpaper" the backside with galvanized corrugated steel, to keep an air gap behind the main structure.
5 years later, no rot, no leak, and not many holes from "missed targets" I hung AR500 steel targets of a basic frame I made. Targets came from
ShootingTargets7.com Very high quality. Veteran owned business. Cheap shipping..