I found a military M16 empty round in my back yard (I live in the country) and i don't know what to do with it
As in an empty shell? If that's the case, you don't have to do anything special, throw it in the trash or hold on to it, it isn't dangerous if it's already been fired.
call the police"NONE EMERGENCY # IT IS IS YOUR LOCAL WHITE PAGES" and tell them what you found if you have not touched it yet leave it where it is
Keep it, that's awesome!
As you say, it's empty. Polish it up and make it a coin collector. When it gets full, take the change to the bank.
Cool. Collect a bunch of shells and make some kind of redneck craft with them.