They do get many from straw purchases here, which is already illegal can get you 10+yr in federal prison.
But also they get a fuckton of full auto guns that are illegal here from the mexican military's corrupt members, and other sources like china, somehow they get explosives too including south korean grenades have been found which is wild, and the best is Operation Fast & Furious, where the ATF just directly sold them a bunch of AKs "to track" and then surprise! "lost" them. They get em from multiple sources.