Mags are a simple pound apiece when loaded. That’s only 17 mags, and 17 pounds, for 500 rounds.
P.S. Notice how the fact he was homeschooled is an issue in the press. He is 24 years old but the press wants to make hay of his home schooling, as though that turned him into a lonely and crazed loser.
In fairness, for 24 hours, he was a muslim on Free Republic.