Just saying, single shot 7mm wouldn't need to be very big, could be completely invisible at night and would get you a better shot than the helicopter theory and would leave an actual 7mm round making it appear like a human sniper instead of some fucking robot bullet developed by DARPA. <insert Lee Harvey Oswald figure here>
Drone takes shot dives into ocean directly after. Evidence? What evidence? To me this is the most likely explanation and why no planefags are gonna show a helicopter there at that time.