The only time I remember a full auto mass shooting was the Bank of America LA robbery in 1997. I never heard how they got those weapons. I would think they weren't acquired legally.
The 1986 law banned manufacturer or importation for civilian use all full auto weapons. The law limited full auto guns to weapons registered prior to 1986 so the pool of weapons in 1986 is what we have today.
Limited supply means very expensive. An M16 could cost as much as $30,000. Obviously to own a pre-ban full auto you have to go through an extensive background check by the ATF, pay the government $200 for the tax stamp, and wait a year to receive permission from the government to own it.