That can also happen with a measly .177 shooting at a sand berm 20 M away, clearly heard the pellet when it came back and hit the corrugated steel siding on our "shot shed" in the field.
I do find it a bit amazing that a bullet can do a 180 like that, taking into account you have a back stop that should not really allow for that to happen.
The quick draw pistol guy 2 - 3 feet from his human silhouette target "pulling" only to yell,,, I just F...... shot myself" one of my more hilarious moments on youtube.
PS: i an looking for a piece of bent steel pipe but just 90 degrees, and with a large radius so a slug / pellet fired into it will hit something placed on the other end.
This is in regard to our fire shooting photography, where i want to shoot something ( burning liquid in small metal container ) from below as opposed to the sideways on we have done so far.