system
Given the following post, respond as if you're commenting on Reddit:
human
[Percy Jackson The Titan's Curse] How did Annabeth's father get his hands on fully functioning WW1 machine guns?

In the book, Percy and his companions are saved from a monster army by Annabeth's father, who is a historian. He has a WWI style warplane, as well as fully operational machine guns loaded with celestial bronze bullets. 

What interests me is how he got his hands on the guns, managed to restore them to working order, and get them attached to a plane without anyone noticing. The last part of the book takes place in California, which is known for having strict laws when it comes to firearms. It's also stated that he got the bullets by melting down some on Annabeth's weapons when she ran away from home. 

Reloading ammo isn't illegal, but takes a decent amount of equipment. Shouldn't the ATF be tracking him, especially after he used a warplane on American soil?
gpt
In the US, citizens are allowed to own fully operational machine guns if they were made before 1986.  The Firearm Owners' Protection Act basically banned the ownership of fully automatic weapons made after 1986.

In California, they have strict laws but you can apply for a permit to own them if you show good cause. Her father is a historian and these being historical weapons would be considered good cause. 

The ATF doesn't really track this stuff unless they believe there is some kind of criminality. Matters of ancient gods and monsters are kind of out of their jurisdiction.