From a policy perspective, I think you will get significant non-compliance. That's why a mandatory ban coupled with a buyback addresses not only the short-term availability of weapons, but the long term supply of weapons as well. In broad brushstrokes, how it works is that in the short term you make semiautomatic rifles with detachable magazines that hold over 5 rounds of ammunition illegal, or at the very minimum, put them in the Class III category. The effect of that is that you'll sweep up those who will voluntarily comply. You couple that with civil penalties, and I think you'll get more voluntary compliance. As for long term - there will be a segment of gun owners who will not comply. You're absolutely right. So what happens is that we make the aforementioned rifles illegal from the legislation's effective date forward, along with certain accessories, to starve the market of new product. As people grow old, die, kids become aware of the rifles, etc., those rifles get turned in. And the ones that don't are effectively frozen in place to their current owners. The prospective mass shooters cannot get a rifle, because the market is generally frozen. Will illegal sales and transfers happen? Yes. Will certain people disregard the ban and keep what they own? Yes. But overall, I think you'd see that the firearms typically used in a mass shooting will no longer be used in mass shootings the farther we get from the legislation's enactment. One other thing: To my knowledge, there are no mandatory buybacks in any states. Please correct me if I am wrong, but the programs you speak of are voluntary buybacks.