The legal market is the source for the black market. There are some ideas out there that could at least reduce the flow of handguns from the legal into the black market. Universal background checks, purchase limits, and a few other tweaks can cut down on that. People who know they are buying a gun for someone who can't legally own one are less likely to go through with the purchase if there's a background check, less likely to get a bunch if it takes multiple trips spread out over time, that sort of thing. Colorado is looking at requiring better securement of guns during closed hours at stores that sell guns, with an idea towards reducing burglary type theft.
I brought these up on this forum once, with all the supporting evidence. I was told that reducing the flow of guns into the black market with just drive the black market deeper underground. Like that's a bad thing?
I agree that limits on long guns don't cut down on the death rate much - but then again, with all the anti-government yahoos out there armed to the teeth with long guns and a stated intent to use them to fight our elected government, my sympathies with long gun owners are very, very limited.