Drones don't use 50cal machine guns. What they use is missiles and bombs, and they've been guided for decades.
What you'd like to do is put a drone in the air where you can lase 5 people, and have it precision assassinate them. Then have it loiter and wait till their colleagues show up, who, by observing their actions, you can assassinate them to.
The size of a bullet means you'd have a lot of operational time would be very long - you could carry hundreds of rounds on a drone, that would still be just as capable of disabling vehicles or hitting passengers.
The effective range of a Barrett M82 (50 Caliber rifle) is approximately 6,000 feet, and that's not shooting straight up.
Point blank (which, despite the common vernacular meaning 'really close', actually means 'the range of the bullet before you have to adjust for elevation') is 1500 feet, past that, gravity begins taking over.
TL;DR, the range of a Predator drone is definitely greater than the naked eye's visibility range, and even with optics, target acquisition is nearly impossible, and even if it weren't, unless it's flying very low relative to its ceiling, you can't hit it anyway.
25000 feet = 7620m, which approximates to 7.6 km ;)
6000 feet ~= 1800m
1500 feet ~= 450m