Full auto weapons are not "banned" from civilian possession. In fact, until 1934, there were no controls on such firearms, when the National Firearms Act was passed, which regulated the sale of such arms, requiring registration and payment of special fees. This was still "bearable" until 1968, when the manufacture or importation and registration of any new full auto firearms for civilian sale/ownership was banned, limiting the availability of full auto weapons to those in existence and registered at that time. The result was, of course, that the cost of full auto firearms simply skyrocketed due to the limited numbers available for civilian purchase. But if you have the financial means (average cost of a full auto today can run $20K-$30K or more) and are willing to apply for a license (really just a $200 stamp, plus all the background investigation, etc) you can still buy any of these registered full autos.
Jim R