The human mind always finds a go-around. California bans 30-round magazines. These clips, hold three 10-round Ruger 10-22 magazines. As of today, they are legal in CA (rapid fire contestants love 'em): 
Let’s consider high capacity magazines.
Suppose, the law allows 30 round magazines.
Suppose, there is a mass casualty shooting, say 30 killed, and we ban all magazines over 20 rounds.
Suppose, there is another mass casualty shooting, with only 20 killed. Would the gun controllers say that saving 10 lives is enough or would they want to ban all magazines over 10 rounds?
Where does it end?
The human mind always finds a go-around. California bans 30-round magazines. These clips, hold three 10-round Ruger 10-22 magazines. As of today, they are legal in CA (rapid fire contestants love 'em):