>with enough agility to thread a needle faster than a turret can swing its own mass
If it has a range of 1.5km, then the drone needs to be moving really fast in order to move a couple of degrees per second.
If it's going like 80mph, then that's 36m/s, which comes out to <1.4 degrees/s at 1500m away. For 1000m, you get ~2 degrees/s. Not to mention that this velocity is at a right angle to the turret, and you have to close the distance.
The point is that you can send lots of tiny drones to converge on the turret. They have to cover 1.5km (though that's only the distance if there's no cover, i.e. the gun is on a perfectly flat plain with no trees, buildings, etc). If the drones are travelling at 36m/s then that's about 50 seconds to cover 1.5km. If you drop 2000 drones, you'd need to kill 4 each second. That seems like a lot.
If it has a range of 1.5km, then the drone needs to be moving really fast in order to move a couple of degrees per second.
If it's going like 80mph, then that's 36m/s, which comes out to <1.4 degrees/s at 1500m away. For 1000m, you get ~2 degrees/s. Not to mention that this velocity is at a right angle to the turret, and you have to close the distance.