My brother and I went thru this same quandary for years and found no suitable portable backstop for shooting any measurable amounts of centerfire ammo. We tried stacking logs, sandbags and even constructed a 4x8 wooden backstop outta plywood and 2x12s that measured better than a foot thick. All worked well for a short amount of time when used occasionally with small amounts of ammo, but didn't validate the amount of work and or expense to build and move when used regularly with large amounts of ammo. We then started shooting from elevated stands and using the ground as a backstop.......this worked well for short ranges, but at ranges over 100 yards, we were still concerned with ricochet. We have since invested in 4 loads of washed sand@ $40 apiece. Two loads were dumped singlely for use for the handguns/rimfires at short ranges and two were dumped together to make a larger berm for shooting at long ranges. Now along with a coupla portable shooting benches we can shoot from virtually any range and direction with relative safety.