The difference between the Hughes Amendment and this prospective ban is in the number of existing weapons. Registered machine guns (in 1986) were in the hundreds of thousands, while "assault weapons" today are in the tens of millions. Note also that in the interval between the time that FOPA passed the House (April 10, 1986) and when it was signed by Reagan (May 19), there was a frantic rush to "paper" as many machine guns as possible. So there was a temporary glut of supply. Prices didn't dramatically begin to rise until about ten years afterwards.