Is the Signal Flare a gun?
Last Night on Earth guns should say "Item - Ranged Weapon - Gun" beneath the picture, and also  have a little red icon of a gun in the top right corner of the text box.

I don't have the card to hand, but my understanding of Signal Flare is that it doesn't conform to the above and isn't a gun (despite the obvious superficial similarities).

It seems a bit strange that a priest would have moral compunctions about using a signal flare in any case!