This article and headline are pretty effed.

Based on the original article they are quoting from (https://www.sfchronicle.com/bayarea/article/roblox-conferenc...), the event was cancelled after someone posted threats online and started an altercation with security.

The fact that he would later be booked that evening with additional firearms charges seems fairly irrelevant and it doesn't even make sense with the timeline as this headline suggests.

tweet thread posted yesterday gives some visual account https://news.ycombinator.com/item?id=37458335

Weird framing; surely the gun he had (also illegally in that state) was more concerning than the magazine? In any case, it doesn't sound like he meant to hurt people. The gun and magazine were in his car and he told police about it after they confronted (arrested?) him for being disorderly, presumably because they asked him if he had any weapons and he answered truthfully.

Given the intersection of "high-capacity magazine", "handgun" and "armor piercing rounds" my guess is he had an FN Five-seven with a standard capacity (20 round, illegal in California) magazine and probably standard 5.7 ammunition (not actually armor piercing rounds, but often misconstrued as such.) Possibly carelessly brought into the state because he's an Arizona resident who normally has a gun in his car. Or maybe he willingly flaunted California's laws, but either way cancelling the event after he was arrested does seem like an overabundance of caution.