You have really strong arguments and remind me why a lot of my thinking has been slightly changed over the years thanks to planetice. While I still think access to guns should be way harder to get (as like I said, my brother can get one and he shouldn't be able to. He can go to a gun show and get one without any hassle, it's ridiculous)... I understand the deal about prohibition as well as anyone else, as authorities trying to keep me personally away from illegal stuff that I want has been a laughable failure. If I want even the most extreme disgusting stuff like PCP or Heroin, I could theoretically procure it within a couple days at the most.
So I do understand that people who -really- want to kill will still have access. But even so, I still think there is absolutely no harm in tightening restrictions up to a degree that makes it annoying for people to get.
Even if it simply has the indirect effect of driving black market prices up, that is worth it. You said that these killers don't have a monetary care - completely true. But that doesnt mean that having a higher black market price is useless. It's very, very useful. A lot of the shooters that have used more expensive semi auto's over the years might not have been able to afford those guns if the BM price was $7,500 rather than $2,000 (for a fking filed AR-15!).
There has to be some sort of merit, no matter how small, in increasing the black market price through any means possible. And cutting off legal production, with exceptions, has to be one way to do it. Am I still completely wrong? I feel this is a common sense way to look at any black market.