San Francisco 49ers linebacker Reuben Foster was arrested on Sunday, and booked on charges of domestic violence, making a criminal threat, and possession of an assault weapon. San Jose Mercury News reporters Patrick May and Robert Salonga are reporting the weapons charge was based on Foster having a SIG Sauer 516 short-barreled rifle.

According to May and Salonga, the alleged victim told the police Foster had, “at least two AR-15 weapons in the house.” Sources told the Merc the police found the 516 during a search of the house.

May and Salonga broke down the details of the weapon in the article linked above, and also explained how it might be illegal in California. California bans certain guns based on a variety of potential modifications. The laws were expanded after the San Bernardino shooting, going into effect this year, but I don’t fully grasp all the nuances of California’s assault weapons ban. Five years ago, Steph put together this rundown of the issue after Aldon Smith’s arrest. It is outdated, but still offers some helpful insight.