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Big difference between what is available and what is not to the general public, semi auto weapons are easy to acquire, newly manufactured auto fire weapons are banned or prohibited, but a person can own one an auto fire weapon, see below.
Despite the Federal Ban, Machine Guns are Still in Circulation
"Although federal law prohibits the possession of newly manufactured machine guns, it allows the transfer of machine guns lawfully owned prior to May 19, 1986, if the transfer is approved by Bureau of Alcohol, Tobacco, Firearms & Explosives (ATF). As a result, a substantial number of machine guns are still in circulation. As of November 2006, the national registry of machine guns contained registrations for 391,532 machine guns.4 In 2006, ATF seized 1,280 unregistered machine guns.5 "
According to msm, this person had acquired guns legally. If this is the case, he must have acquired the machine gun before 1986, or, he altered a semi auto rifle making it an auto fire rifle by purchasing the part/s to do so via mail order or internet. Authorities have not stated the type of weapon/s used in his assault.