I have, and I knew that, I just forgot.
In theory, it's perfectly legal to own sawed-off shotguns and fully-automatic firearms if you purchase the required stamp.
However, purchasing the stamp is an open invitation to the BATF to send a SWAT team to your house at any time.
And if you do not buy the stamp, it is an open invitation for the BATF to send a SWAT team to you home (compound) and start shooting with those same weapons. Do you remember an incident in Waco Texas that occurred about 10 years ago. The lack of a $200 tax stamp was the prime reason for the search warrant (to look for a gun that might have required that tax stamp).
The Marijuana Tax Stamp Act was modeled directly on the Machine Gun Tax Stamp Act. Basically the practice was this: require the Tax Stamp, but refuse to issue it.