What we now call drones were round basketball shaped flying machines that would shoot enemy soldiers, but not friendly soldiers. The big secret was how they knew which was which. The big secret revealed at the end of the story was that they shot (bullets) at any one that shot at it.
Sounds a little like screamers http://www.imdb.com/title/tt0114367/?ref_=fn_al_tt_1
There is a special device they use to not be attacked and I'd say some of the screamers we see are about the size of basketballs. They don't actually fly, but there are scenes which could make you misremember flight capabilities.
Best I can do as the question is written.