This is a flawed analogy. A breathalyzer completely prevents a drunk from using a car, while allowing those who are capable of driving to use the car without restriction. What you're proposing by limiting magazine size is more like limiting all car gas tanks to 3 gallons, regardless of the driver, because if you need than 3 gallons to get where you're going, you obviously don't know what you're doing.
Either way, it should be obvious that there are no good analogies for the gun control debate.
The debate is far, far more complicated than {more gun == more murders}. Frankly, there are no truly good models to look at. Americans are different than Europeans, and Australians, and Mexicans, and Canadians. I'm not being racist, or nationalist, the fact is that every culture behaves differently. For example, it is often said that most of the guns used in Mexico are purchased in the USA, making it seem as if the US's relatively lax gun control laws cause the crimes committed in Mexico. This is a fallacy. Canada also borders the US, and the Canadian border is nowhere nearly as controlled as the Mexican border. Yet, Canada does not have a massive violence problem cause by US guns. Canada is, overall, a pretty safe place to live. So, the logical conclusion is that there are far more complicated forces at play.
Canada has a universal health care system, the US has a jumbled mess of different systems and compromises. Perhaps this has something to do with the lower violence?
Canada also has a lower population density. Perhaps that is also a factor?
Frankly, in my opinion, America has a health care problem. Specifically, a mental health care problem. It's hard for those who need help to get it. There is a very negative stigma attached to discussing any type of mental health issue. People will think you're weird, or strange, or dangerous (maybe in some cases you might be). I would wager that there'd be a lot fewer suicides if people weren't afraid to talk about being depressed. It's not about where guns are available - look at Japan, they have a lot fewer guns than we do, and a lot more suicides. It comes down to culture. I comes down to health care. It comes down to the stigma on mental issues. It comes down to the availability of weapons. It comes down to the response speed of the cops. It comes down to poverty. It comes down to class issues. It comes down to racism. There is no simple answer. Life is complicated, people are complicated, and the world is complicated. Make an effort to see without the blinders of fear on your eyes (oh no! Guns!).
I'm a gun owner, and I'm not saying that you're definitely wrong. Who knows, maybe reducing the magazine size will reduce the rate of violent crime? But maybe, just maybe, the issue is far more complex than you think it is. Who knows, maybe the gratuitous violence that tends to pervade parts of our culture is actually a factor? But I would wager a bet any day of the week that a change is the way we handle mental health issues and how we deal with poverty will produce a larger change in violent crime than just about anything else.
Try not to be so narrow minded. Before you attack anyone else's rights, why don't you take a look at the killers, instead of their tools. Because, let's be honest, if someone is seriously contemplating killing as many people as they can, but is unable to because they don't have the tools, we've still lost. It means that our society has produced someone so screwed up that they just want to kill people for no particular reason. Even if they can never accomplish their goal, something is seriously, seriously wrong.
Look at the killers. Look at the traits and issues they have in common. Then, try to make an informed decision.