If someone wants to kill lots of people, said person will kill lots of people, regardless of the laws. Making guns harder to legally be obtained is not the proper way to go about it, because there are ways of illegally obtaining guns.
This is a slightly fallacious argument. The same could be said about laws against murder, but nobody wants to get rid of murder laws.
Every time you ban something, you're going to decrease the amount of people doing that thing by some appreciable amount. You're adding a negative incentive to that activity, and people will respond by doing it less. This is true for almost everything.
You have to balance whether the regulation is worth that decrease though. For instance, the war on drugs is an example of terrible regulation because it costs us tons of money to incarcerate otherwise non-violent criminals. The decrease in drug use as a result isn't that significant either.
My view on weaponry is that we should add harsher regulations for weapons that have a higher propensity for murder and a lower propensity for recreation/self-defense. For instance, hunting rifles would be less regulated, while hand grenades would be pretty much banned.
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