Not too long ago I had a very eye-opening experience during an afternoon plinking outing. We were shooting short range at a woodpile at the bottom of a substantial hill. I was firing different rounds (mostly subsonic) through a chronometer – slow pace, very controlled fire, and could account for every hit.
Afterward I checked in with my friend, who generously allowed us to shoot up his woodpile, and he told me he could hear ricochets flying over while working in an outbuilding on top of the hill (not at the edge of the hill but more like 30 feet in from the edge).
Woah. That may come as a surprise to you- it sure caught me off guard.
The problem is velocity– subsonic ammo isn't moving fast enough to split or flatten projectiles. This leads to slight deformation of the projectile and subsequent tumbling. They end up bouncing around like “rubber balls” according to some associates interviewed in my research.
This is not completely intuitive because the sound of the shot going off masks the sound of the ricochet. Supersonics are probably doing it too sometimes but the SMKs are really prone to ricochet.
The fact that a woodpile isn't a sufficient backstop was another surprise–my research into this topic has shown that railroad ties and in some cases dirt birms aren't always enough either.
Bottom Line: be sure to know and follow Cooper's 4th Law: know your target and what lies beyond it. Please be safe out there.