From the deceptively named Firearms Owners Protection Act:
“The Act also contained a provision that banned the sale to civilians of machine guns manufactured after the date of enactment, restricting sales of these weapons to the military and law enforcement. Thus, in the ensuing years, the limited supply of these arms available to civilians has caused an enormous increase in their price, with most costing in excess of $10,000.”
the 1986 Act also put draconian limits on suppressors and imported gun parts