There really isn't a market for illegal full auto weapons here in the US, but you ban semi-auto sporters, you will open that gate. Not so much the demand for full auto, but the black market for illegal guns will drift to elements who could provide stolen or surplus military weapons easier than semi-auto.
"In the world I see you are stalking elk through the damp canyon forests around the ruins of Rockefeller Center. You'll wear leather clothes that will last you the rest of your life. You'll climb the wrist-thick kudzu vines that wrap the Sears Towers. And when you look down, you'll see tiny figures pounding corn, laying stripes of venison on the empty car pool lane of some abandoned superhighway." T Durden
"In the world I see you are stalking elk through the damp canyon forests around the ruins of Rockefeller Center. You'll wear leather clothes that will last you the rest of your life. You'll climb the wrist-thick kudzu vines that wrap the Sears Towers. And when you look down, you'll see tiny figures pounding corn, laying stripes of venison on the empty car pool lane of some abandoned superhighway." T Durden