I have a 230 grain ball slug from a 45 acp somewhere at the house that hit me in the ribs. Many years ago, when I was in the service, we were conducting a qualification shoot and one particular individual couldn't hit the backstop let alone the target, as I was getting to him to get him stopped and corrected I saw the dirt puff 15-20 feet in front of the target, the dirt on the backstop puff low and higher and then about halfway up, and then a flicker near the railroad ties as timbers over the range to keep bullets from getting out. It worked, the bullet didn't get out, it hit me immediately back left of the shooter right under my shirt pocket hard enough to make a welt and a decent purple and green bruise the next day. It was laying at my feet when I looked down. He got pulled from the line and didn't shoot again. Bullets, especially when the speed gets low, do very weird things. I saw many tens of thousands of rounds go down range at that particular range and never saw anything to indicate that a bullet had even traveled part way back from the backstop before or after. I never worried about it going back to that range, what are the odds that it would happen once let alone twice?
Back to the OP question, spending lots of time and working matches at ranges over the years, the best bet is angle iron with the angle to the firing line, with tabs on the back to hold half sheets of thin OSB. 3" angle is plenty big and heavy. Anything that hits the angle will either deflect to the side into the berm or go through if a high powered rifle and they can take a LOT of hits before needing replaced. I have seen timbers placed in front of the angle too, theory being they will catch or slow down anything headed for the angle iron and it's easy enough to angle the iron and/or the timbers slightly toward the firing line at the top, in theory deflecting anything that might bounce off down into the ground. The OSB gets replaced periodically and it's as close to dips!$& proof as I have seen.
