If your getting 7 bb's at 40 paces( roughly 50 yards??!!), that is awesome for a 16 ga, heck, for a 12 guage thats awesome!! Even if you dont hit him in the brain, and you get 7 in the head, that will break his neck, but chances are, your will put at least 1 in the skull. I would have to say that that will be plenty of knockdown power, if you want to be sure, upgrade to the federal HEAVYWEIGHTS, a # 5 or # 6 (equivalant to a #3 and #4 lead weight-wise), and you'll get more bb's that are heavier, and carry more energy further.

Good luck