well, some indoor ranges use metal to deflect bullets into water below it.. this makes it easy to retrieve bullets. You might call some ranges about what angle, etc. My guess would be that the angle is 45 degrees (I think this is the angle of the BB traps they sell). denting the metal should not be an issue if it is thick enough.. especially since you are only using 22 shorts (after all.. the indoor ranges allow much more powerful rounds).
I can vouch from experience that wet newspapers or phonebooks work well, though. And dirt is a proven military backstop..