Personally, I've never stood by the idea of hexing guns. Truthfully, if you can hex a gun, what's stopping that same hex from fouling up all the doorknobs in the same zone (mechanically speaking, one is about as complicated as another)?
Besides... in the books Harry tends to sling around a lot of magic, but still those pesky bad-guys seem to be able to sling bullets his way, shouldn't the "Angry Wizard who is now full of holes" aspect be compelled to jam their guns? But this never happens in the stories, he always has to find a way to separate bad-guy and gun.