However states like California with very strict gun laws have very high murder/homicides by guns. The theory at first blush seems plausible however it is just that a theory. The numbers say otherwise.
Chicago and New York are also good examples. You just as easily acquire this RPG everyone has been talking about as get a gun in the city limits of New York legally.
Outside of highly sensationalized and emotional mass shootings most of the death by gun is going on in the inner cities where gun restrictions are highest.