Well first, this may be more opinion than anything. Have read of an officer killed at 85 yards. One pellet entered his eye and killed him. Know of deer killed at 80-100 yards. Again, usually freak pellets entering an eye, ear or something similar.
Considering how it it loses velocity, 50 yards is likely stretching it. 25-35 yards is where it probably hits the limits of good effectiveness.
Wish I had the links, but someone had done ballistic gelatin testing at different ranges. And beyond 35 yards the amount of penetration was way down.