Lots of people and money
I can easily believe that CA represents the largest U.S. state market for guns and ammo. The state is the largest in the U.S. with respect to population, having over 30 million people, which is 10% or more of the U.S. total. Combine this with the higher average income in California than most states, and you probably have about 12% to 14% of the potential market for any consumer item. So you would expect guns, a somewhat expensive consumer item, to sell pretty well.
I kind of doubt that California is 25% of the U.S. gun market, however - that sounds too high to me.