I see what you're saying, but the fact is, Rowling didn't create a complete setting, and it allows for other locales, rules, attributes, and features, so long as they fit within the initial setting. All tabletop gaming in a pre-created setting is predicated on that. This particular world is identical to our own, just with the inclusion of magic. Other than that, the physics are identical until the primary source explicitly says otherwise. In the case of guns, they are confirmed to exist in-universe, and they're confirmed to work just fine under ordinary circumstances, the kids just don't/can't get their hands on them. Now, they might not work on Hogwarts grounds, but there's no rule that says the tournament MUST be held there, only that it was held there that one year.