Hay bales make great "first line" bullet slowers/stoppers. Put something solid behind them (steel, dirt filled tires, concrete etc), esp if you're shooting something like 22 mag or rifles. I've seen round bales, on edge, reliably stop and trap 44 mag and smaller for at least a year in the weather. Replace when you feel the hay is approaching the fail point. I'd think a squeeze of hay (60-80 square bales) would stop essentially anything. That's about 8x8x8 ft. I'd still always want to back it with something solid for the just in case factor.
Sgt Lumpy - n0eq