ARs sold in California are limited to a ten-round magazine and must be designed to require a tool in order to change magazines, as opposed to a simple button press. In practice most manufacturers have dealt with the latter requirement by using a "bullet button" that can be pressed with a bullet tip (a bullet constitutes a "tool" within the meaning of the applicable CA law). One could easily modify that to not require a bullet or other tool (it could be as simple as a piece of plastic that would fill the bullet hole and extend out to form an external button), and there are commercially available kits to do precisely this. As for the magazines, 30-50 round magazines are readily available anywhere in the US other than, if memory serves, CA, MA and NY, so the shooters could easily have bought some in Nevada or elsewhere, or had them sent to them. Modifying the bullet button and using 30 round magazines would be illegal under CA law, but so is going on a killing spree . . .