These kinds of laws are already in effect in Chicago. Guns come in freely via Indiana where it’s constitutional carry, plus other means. It turns out that banning guns only means that law abiding citizens — which as the above commenter pointed out, statistically almost never use their weapons criminally — stop carrying and criminals have carte blanche to go target practicing on defenseless meat bags.
CA is a massive state, and I’m sure it has plenty of weapons. Criminals aren’t going to magically stop committing gun crime because of a stupid law, and if anything it will only embolden them as they’re less likely to encounter a CC citizen. Guns will continue to flow in via NV, AZ, and Mexico.