I have noticed it but it gives me no trouble. I am not hitting head shots 99% of the time, so the interaction of the two together does not bother me. But the explosive perk can lead to firefly not activating. That being said, I use this almost as a mid/long range crowd control. The explosive rounds just demolish mobs. And if the firefly perk activates, even better. I would level it at least until you have to start giving up energy.