It depends on the gauge of shot. It also bears mentioning that having metal shards raining down at an airport presents a substantial FOD risk to engines and could also be kicked up by tires and strike the wing and body.
You overestimate the strength of aircraft exteriors. Even minor scratches become important under full stress.
#7 or bird shot would not even scratch the paint if falling under gravity. It is approximately 0.1 gram. If you ever see what it looks like in real life, it is tiny and soft. The only reason it does damage is because the charge gets it going at a fair speed (which is loses very quickly).
Since you seem to have experience here, what do you think the lightest shot that would damage a midsize drone (ie DJI phantom) at a useful range would be?