Pretending to be a Sniper
This guy was playing with an Assault Rifle, but he suddenly switched to a sniping rifle for his final killcam shot.
That is a good way to fool others into thinking that you have played as a sniper.
Till now, I have not seen anyone who made his 30 kills in FFA using a sniping rifle. I believe it is related to the maps in BO3. Generally the maps are all very tight and without a good long range line of sight.