It only applies in the urban services area of the county. Basically, if the city picks up your trash, then no BB guns, etc. If you pay a 3rd party service to get your trash, fire at will!
Urban Services vs. General Services was how Nashville has tried to balance the county-wide "metro" government with the fact that some areas were and still are fairly rural, especially in the far west and northwest parts of the county. Up until just a few years ago, there were cows grazing on land next to Opryland along Briley Parkway. For civil society, the council wants different rules where population density is higher. Things like noise ordinances, and shooting guns, for instance need to be different in say downtown vs. a 100 acre farm near Joelton.