I have no experience with any AR-xx ammo penetration, however I do with many other calibers concerning sand. My backstop was built in 1972. It is a number of railroad ties laid in a shallow cup shape and stacked similar to a split rail fence and backed up by 4 wooden utility poles set in the ground behind them. The face of this was covered with corrugated plastic sheets and then 2 dump truck loads of yellow sand was piled in front of that wall. At the target line the sand is 4 to 5 feet thick and about 6 to 7 feet wide. Yellow sand in this area is abundant, economical and does not contain rocks.
Many different firearms of various calibers have been shot into it over 50 years. A typical jacketed hunting bullet from a 30-06, from 90 yards, will penetrate 7 to 8 inches depending on type. FMJ ball will go deeper. I have found .45 caliber muzzle loader lead bullets, also from 90 yards, elbow deep in the sand.
As for maintenance, one time about 10 years ago, the sand had settled some and I had to shovel some from the front back up on top of the pile and replace the railroad tie that is below the target line that keeps the sand in place. Full disclosure the local woodchucks were responsible for much of the sand movement and they make interesting targets of opportunity.
If there is a possibility of upgrading your firearm collection, a foot of sand may be a problem. Plywood will not last long exposed to the elements, and after a bunch of holes it could be very difficult to replace easily.
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