Little data exists but in theory lead free munitions don't contaminate soil. The inorganic salts in gun powder easily dissolve in water and the NO3 is actually a nutrient that can feed plants and micro organisms. The carbon and sulpher aren't as easy to get rid of. Carbon is of little concern, sulpher could be in high concentrations, and you can never tell the impact on down chain ecosystems especially if the residue leaches in significant amounts into the water table. I suppose the next time the gun debate comes up and anyone says "guns are only for killing" a cheeky person could argue they can be used for agriculture too. Of course you could probably do as much good shooting a bag of seeds, fertilizer, and mulch with a regular shotgun shell. Art. Go figure.
*pew pew pew*
Police: What are you doing?!
Me: (probably not though irl) Gardening
Wondered were C&H got the idea