B
Therein lies the question Are they getting them legally or are they getting them under the table so to speak? If they are getting them on the black market then they are already skirting the law. The issue then is not the laws itself but how do you deal with those breaking the law in the first place?
Take drugs for example. With your logic, there's no point in making heroin illegal because they'll still have access to some on the black market.
We need to prevent more horrible with mass-shootings, and one way is being sure that the people who have things that can kill a lot of people are known about!