Most people would peacefully turn their guns in instead of risking jail time for non compliance.
maybe you're right and only 40% of them of refuse. Then what? Do you think those in the 60% rolling over like cowards will be the kind of gun owner that "stockpiles"? Or will it mainly be people that inherited a gun 15 years ago and never used it or even bought ammo for it, does it even work? Nobody knows. The guy with 400 guns isn't turning his guns in. None of the criminals are turning their guns in, there will be no difference at all in the number of guns criminals have.
Sure, in a few decades as the bad guys slowly get caught, grow old and die and the guns slowly stop functioning due to time and poor maintenance, yeah, eventually the numbers of guns will begin to drop a little. On the other hand, some bad guys don't get caught, some maintain their guns and sell them when they get out of the business. And ya know, gun running is big business....perhaps Trump's wall will help there....wait, what?
So "most" gun owners turned their guns in and it only got 18% of the guns, most of those being old, broken or paintball guns. Now what? Does anybody think crime or even gun violence would go down then? Maybe in 10 to 20 years, in a perfect world....