Best homemade backstops....

[txgunner00]

Stack the tires in a double row with the centers shifted as you go up. Fill each layer with dirt Then the bullets won't go through and the dirt won't go away.

I've seen them used like that in a Sheriff's training range in the Sierra Nevada foothills.

This. Double row of tires laid up like brick. Dry sand is the best material for stopping bullets. Whatever on site soil mixture you have is usually most practical and works nearly well as long as its not too rocky. Gravel and rocks tend to send bullet and rock fragments flying. However, it usually won't matter being contained in tires. Sand bags are a different story though.