The size of your berm should not just be sufficient to catch the bullets that go right through your target in the scoring rings,
but the berm / backstop should be wide and tall enough to catch the bullets that are fired when you, or your trainee, or your guest, puts their finger on the trigger the moment they put the gun to the shoulder and fire before they even acquire a sight picture.
You could fill a cardboard box the size of a microwave oven with dirt or even cat litter and use that for a backstop --if (if) you knew that you wouldn't miss. But you should take into consideration the possibility of the gun firing without having a proper sight picture, or you do carefully aim the rifle and take the shot, but you miss by a foot because you didn't realize that your sights or your scope had been bumped recently.
but the berm / backstop should be wide and tall enough to catch the bullets that are fired when you, or your trainee, or your guest, puts their finger on the trigger the moment they put the gun to the shoulder and fire before they even acquire a sight picture.
You could fill a cardboard box the size of a microwave oven with dirt or even cat litter and use that for a backstop --if (if) you knew that you wouldn't miss. But you should take into consideration the possibility of the gun firing without having a proper sight picture, or you do carefully aim the rifle and take the shot, but you miss by a foot because you didn't realize that your sights or your scope had been bumped recently.