Welcome aboard!
You really HAVE to dig that shotgun shell brass. . . depending on soil conditions, depth, etc, those shotgun shell targets could easily be old nickels, gold, or vintage CrackerJack prizes from the early 20th-century. BTW, there's a guy on the forum (pretty sure it's "turtlefoot13") that's putting together a database of the head-stamps on shotgun shells. I've been sending him all the brass that I find and after he dates it, I have a good idea of the age of the sites I'm hunting.