In Percy Jackson and the Titan’s Curse, Annabeth’s dad, Dr. Chase comes in at the end of his book and uses a machine gun on his Sopwith Camel, along with some celestial bronze bullets he created in order to pulverize the monsters that are closing in on Percy and his friends.
In addition, Luke has a sword named Backbiter that is half steel half celestial bronze, so that he can kill both humans and monsters. Also, Chiron says that Demigods are vulnerable to both weapons aimed to kill mortals and weapons aimed to kill monsters.
So, at the Battle of the Labyrinth, Percy and his friends could have decided to defend camp with a bunch of machine guns loaded with celestial bronze bullets. Also, when Percy and his friend Beckendorf decide to raid Kronos’s ship the Princess Andromeda in Book Five The Last Olympian, why couldn’t they have carried guns? It would easily do away with all the sword fighting skill needed and it would just be “pow” and Luke is dead…
Clearly, there must be something wrong with my logic. But, is there any drawback in demigods using guns in general? What might prevent them from doing so? I'm looking specifically for practical reasons, but any reasons are fine, too :D
EDIT: In response to @JBH, I am looking for anything, even outside the Percy Jackson Universe. Percy Jackson was the only series I have read that included Greek Demigods, so I used it as the majority of my example in the beginning. Thanks to JBH for proposing this clarification. *