Forget slant range... and other classroom garbage...
Just by eyeballing when the flir zoomed back out:
Its in range... (fast moving attack aircraft going 400mph+ with escape vector then you start talking effective range)
These weapons go a long way.. Effective range is just that against fast movers..(note unclassified) against slow movers without getting into details it is much greater hence the CIV air threat manpads dictate.
Slow moving thing = no chance... and considering the angle you keep expressing higher angle is bad for the target, less escape vector. (thats is unless your in a f15/f22 and just point up and out energy the thing. UAV does not have that capability.)
Quick exp:
Effective range on a m16a2 is about 550m while the .223 goes a hell of alot farther... 3600m
Draw your own conclusions, Lucky shot

DoctorYo