They aren't any more vulnerable to bullets, than to anything else. Any force field will deal with that.
Mostly the reason individual drones can be shot, with drones' defences only appearing later, is that the hive mind just doesn't think much about individual drones, or the safety of one at a time. Sometimes they have trouble adapting to a weapon, but not for long. A few seconds?
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You can shoot a few drones down at first, because the hive just isn't thinking on the level of what's going on with the ants in the ant farm, and that includes their own people. The whole has to be hurt in a bigger way to make it sit up and pay attention ... and then bother to "adapt" to a weapon.
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Maybe the idea is supposed to be-- can't they take out hundreds in just seconds, unexpectedly, with automatic weapons? Like Picard in FC? For a few seconds you get somewhere, but once the hive notices you, they stop you. Any standard force field would be tried as a default, I'd think , and that's that. So it c an help one man like Picard get out of a jam fast an d escape, but won't help much o n a larger scale.