Among the gun control provisions passed last year that went into effect at the beginning of this year was a restriction on the sale of ammunition. This is plainly a means for CA to control (and restrict) ammunition sales in the state.
http://www.breitbart.com/california/2018/04/27/nra-sues-california-ammunition-controls/
On January 1, Breitbart News reported that new California state laws bar out-of-state ammunition purchases and require that all in-state purchases be made from a state-approved, licensed dealer. This shrinks the supply, which will inevitably drive up price. Moreover, they require that any ammunition purchased online be sent to a licensed in-state dealer, who will then charge a processing fee for the ammo, thereby driving the price up even further.
http://www.breitbart.com/california/2018/04/27/nra-sues-california-ammunition-controls/