Rifle and pistol rounds do not ricochet off dirt very high. A ricochet (contrary to what seems to happen in the movies) is a 'bounce' and loose dirt or somewhat packed dirt isn't very bouncy. A stray rock in a not so opportune place WILL cause a ricochet, based on the relationship between the path of the bullet and the angle of the immediate face of the rock. And since one cannot figure that ahead of time, it's anyone's guess.
Probably you have already hit on the best way to control stray rounds. Limit the use of the range to people you know to be rational. This probably will annoy some of the semi-automatic guys who want to reenact their favorite John Wayne movie, but life is hard.
Be discreet.
I've seen a projectile than ricocheted off the ground in front of a target, ricochet off the berm behind it because of the low angle and the distance between the target and the berm. Myself, I would rely on one very high berm and use portable benches and pop-ups for shade and rain protection when I wanted to change shooting distance, instead of several smaller, less effective berms. I am lucky, my personal range is in a valley that is surrounded on three sides with very steep bluffs on three sides. In order for any stray round or ricochet to get out of there would mean it would have to have a very steep angle up, meaning very little or no chance for it to get to anywhere someone else could be. That doesn't mean folks wouldn't be there. That is what one needs to accept. Regardless of how you plan, you cannot plan for what others may do. Have had folks that were not supposed to be there walk out after we were shooting because they were lost(or so they said), hunting game or edibles(again, no permission), or even folks with permission, that did not inform us they were there. Thus, the first few rounds are always shot at the closest target with the bluffs directly behind it.
I also shoot at a small, local sportsman's club range. Been there since right after WWII. Odds are it would never be allowed to be built where it is now today. Mostly used for trap shooting and for sighting in deer rifles before season, but in the last coupla decades, used a lot just for recreational shooting of high capacity handguns and rifles, by folks that really don't know the potential. They just assume that it's a gun range and any shot fired there should be safe. It's not. Local road directly to the north has a lot more traffic. Homes have been built downrange. While they cannot be seen because of the distance or the berm....they are still there. The forest of trees that used to be heavy, has been thinned by cutting and disease. Anything not shot directly at the berm has the potential to get out and does. What I see going on there from some shooters scares the 'ell outta me and I assume it's just a matter of time.
Thus, I totally agree with the idea of limiting the use of your range. Not necessarily limiting the use of semi-automatics, because assuming those folks are less responsible and safe, is in itself, irresponsible. Just know that those folks shooting are doing it in the safest way possible. Folks new to shooting need to be watched carefully and the firearms they use limited, until they are familiar with them. Being discreet is impossible whenever you shoot more than a few rounds anytime but during hunting season. Anyone within hearing distance will know what and where it's going on. This can be a good thing or a bad thing. At my range, it's hard to shoot for any length of time without a neighbor showing up either to watch or join in. It becomes a social thing, which I enjoy. But one still needs to be prudent about what is being shot where and not to allow alcohol and firearms to mix.
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