Weird, it works perfectly fine on my end. Actually they tend to not last long at all. I just double checked and the bounds are fine.
However, I don't like those drones much, they will probably change someday.
I'll take a screenshot so you can see what I'm talking about. You are right though, they are very frail and don't last long once hit. And even with what I've experienced, they aren't overpowered. But it does feel like they distract PD weapons longer than they should. Anyway, the screenshot is on its way.
And let me stress, Scy's balance is superb and the mod is very enjoyable!
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Not the best example, but it should give you an idea of what I am talking about. Lots of weapon projectiles moving through the Hullbreakers without hitting the drones. Again, I know it looks like the PD weapons are grazing the side of the drone in this screenshot, but I've experienced the same effect with the weapons practically hitting the centre of the drone. In fact, if you look closely you can see a number of these rapid fire PD bullets have grazed the centre of the drone.
I feel that the Hullbreaker drones need slightly larger bounds. The red lines are placed a little too far to the right. If you line them up with the projectiles, they actually hit the drone's sprite 100%.