User blog comment:Miskos3/Fairy Tail 529: Teacher, Review/@comment-7418318-20170401141434/@comment-7418318-20170401165922

I guess it might still sting a little even if doesn't do any actual damage and that's why Acno avoids it.

Or he might be worried that the bullets might hit his stump. That place ought to be more vulnerable than his regular scales.