I found out while shooting my 45 at 100 yards this last weekend that a 230 grain projectile fired from 45ACP casing will get through 1" of plywood + 4" of pine boards. I found one bullet lodged on the back side (went fully through and was exposed) when I clipped the edge of the target stand at our range.
Not that good, shooting at 100 yards with a Glock 21. But I did mange to score some hits! 17 of 72 fired landed on the 15" square target... not good ratio, but better than I thought I'd do. Was firing offhand from a square stance.
Anyway .. would recommend stacking old railroad ties and filling it with about 2' of dirt. We did this on a friends land, and it's stood up to THOUSANDS of rifle rounds now. (Basically you build two "walls" of ties a couple feet apart, bind them together to form a "box", then fill with dirt. It'll last a long, long, long time)