Soft pellet @ 1200 fps, I thought it would completely mushroom and drop, sure glad it wasn't a hunting plated one. That was a 50 ft trip bouncing off 2 solid objects. My point - you DON'T know what is going to happen. If it was a keyhole in wood, the bullet would be stuck sideways in the wood plank, right - or a really big hole? I would suggest using fiber wall board for your backstop. Only time I've seen a keyhole was a sub-sonic 22 out of a barrel with the wrong twist. Maybe you had a real light load. A blown up bullet won't give a nice looking imprint. Just be safe.