Cid
Proper break this time
They probably can't hear the drone so as far as they are concerned it could be anything going bang, mortars, grenades outside the window so safer to stay put. Tanks are loud.
Also hunters use shotguns for flying birds as a general rule because it is very, very hard to hit a flying object with a rifle. Like you say maybe 4 or 5 of them firing together may stand a chance but it's not easy. Hence AA guns having such high ROF.
If they had have realised that it was a drone attacking they would have closed the hatch you'd think...which brings us right back to your first point of some very dumb things being done (that we see).
Also dunno how much small arms ammo your average tank crew carries, especially in light of rumoured shortages.