There's a problem with your "if" : the goal isn't to limit the number of firearms available to crazies. If the crew from One Flew Over the Cuckoo's Nest took over the NRA, few people would notice... The issue rears its head when firearms generate victims. Here's the thing. The number of guns in the united states has been going up for decades, faster than the number of people. The number of gun homicides per capita, however, has been going down for just as long. Would lowering the availability of guns drop it faster? Maybe. Probably. Definitively? Decisively? I don't think we can say for sure yet. The problem isn't guns, it's people that think guns solve problems.
Actually, I disagree. My plan is to get rid of the guns people do not value highly, and to give them something they place a higher value on. Guns they are willing to turn in are also the guns that they might sell second-hand, or secure less carefully than their treasured (__________). Sure, there will still be a black market, but why don't we incentivize people to get rid of those weapons in another way? Give them 200% retail value if they melt it down, instead of 10% of retail value to that dodgy dude on the corner, or in the back room of the gun show. Let's decrease the inventory of these guns first. Then we can deal with helping people make better decisions about safety, storage, and appropriateness of any class or type of weapon.
Let's say you have an old Mauser in the closet and you've been eyeing a .223 Savage because it's deer season. Gun buybacks are the best thing that ever happened to you. Thing is, you're exactly the wrong person we want a gun buyback to target. We want that guy with the .38 special with a burned serial number who got it off a buddy that took pity on him when someone on his block stepped up and he didn't have a piece because he's done two bids for domestic violence. That guy? He ain't coming near your buyback. I'm not opposed to buy-backs on principle. They've been tried several times in the US, seldom without controversy. The fundamental problem, as I see it, is they're basically a stimulus program for sporting goods stores. You're effectively subsidizing the purchase of better guns, not reducing the absolute number... unless you also do something to curtail the availability of guns. I'd also point out that it only takes one gun to kill a whole buncha people. Would Sandy Hook have been appreciably less deadly with one gun instead of three? There were only two at Virginia Tech. Meanwhile, over 1700 rounds of ammo went flying at the North Hollywood shootout and there were only two fatalities (one a suicide). Back in my hood, Ricky Abeyta killed seven people (including a 6-month-old baby) with a single handgun. For a reduction in guns to equal a reduction in violence, you'd need statistics revealing that the availability of guns is correlated with the number of murders by guns. I haven't seen data that indicates that, although I've looked into it a few times. The correlation is muddy at best and controversial at worst.
Good catch. You are right... I would first pass legislation that made gun buying/licensing more like buying and driving a car. License, registration, insurance. Same as a car. Then the buyback. Don't wanna pay to have that shitty old Mauser registered and insured? Ok. Get cash for it (or a tax break). Hell, it could even be sold as a stimulus package.