I get that this is not entirely serious, but it's easy to build a case for why muggle weaponry would not pose a serious threat to witches/wizards in the harry potter universe. the series makes several references to passive defensive enchantments. if the entire campus of hogwarts can be made almost impenetrable to hostile magical forces, it doesn't seem like much of a stretch to imagine that robes could be enchanted to be bulletproof. a skilled enchanter might be able to create passive defenses against most/all conventional weapons.