But, ultimately, these laws are as much about changing social norms about guns as they are about the utilitarian objective of reducing the number of people who die in already extremely rare mass shooting events.
These laws add moral authority to a proposition that is very widely felt:
Civilians who feel the need to buy large capacity magazines for their firearms are paranoid or diabolical, dangerous people with a highly elevated chance of committing horrible atrocities (no matter how low the chance of this happening is on an absolute percentage basis). Buying a large magazine is something that only someone who has illegitimate uses for a gun finds valuable and we as a society should delegitimatize you and the uses you contemplate for your firearms when you buy one.Similarly, a law like this legitimatizes the sentiments of ordinary non-gun nut individuals, even gun owners, who feel that someone who feels the need to spend money to buy a large capacity magazine for their firearm is a dangerous individual whom we are right to fear, despise and ostracize. If you have a scary neighbor who also happens to own a large capacity magazine for his firearms, he is probably someone in the community that local law enforcement needs to be as worried about as your average local felon on parole.
Most gun control in the United States is social and not legal. Something on the order of 80% of adults in Colorado have the legal right, with only minimal bureaucratic hassle to get a concealed carry permit and bring a handgun with them to work, while out about town running errands, into stores, in movie theaters, and so on. In a typical urban setting in Colorado, the percentage of non-security professionals who actually do so is under 0.5%, even though roughly half of adults have a gun in their homes, and probably more than 40% of households have a handgun in their homes.
Passing a law like a state ban on large capacity magazines is a way to develop the social norm that paranoid or diabolical gun nuts are a tiny, illegitimate and dangerous outlier subset of our nation's population, despite the fact that they are very politically vocal. By doing so, their influence in the gun control debate down the line can be reduced. Indeed, delegitimatizing these group of people, all by itself, may have the helpful effect of encouraging ordinary citizens to make law enforcement aware of these people, which may very well prevent gun violence by them.