[kraigwy]

I'd stay away from tires, First they cause some wierd recochets, and if you bury them, they will eventualy float to the top.

Our rifle range in town was closed down because the the recochets. The pistol range, that just had an earthern berm was allowed to stay open.

Dirts the best, better yet, dirt with grass planted on it to keep it from washing away.

I'm lucky, I have a mountain as a back stop for my range.