I just put up a metal barn. I want to practice in it, but not through it. What would be good to use behind the target as a backstop.

not missing the target.

Lol, in addition to the helpful post above:

Several layers of carpet draped over a 2x4 "tower," kind of like so:



Or

A hay bale turned sideways



Or

Several layers of this:

Hang an old piece of carpet behind the target.

Actually Savage has a good idea, too. Keep your shooting distance short enough that you don't miss. If you miss then shoot closer till you don't.

I know a guy who has thick pieces of styrafoam(sp) that he found

Try your system outside your shed first. Then move it all in. I think carpet would be the best backdrop.

"loose" carpet is the key. This will absorb the energy. Tight carpet, the arrow will just go through.