Any firearm that fires more than one bullet when you pull the trigger is considered a “Machine Gun” under US law. It doesn’t matter if it fires in bursts, if its full auto, or even if it fires out of multiple barrels. People are always looking for different ways to get around this restriction and increase your rate of fire, without actually turning into a “Machine Gun” and having its sale restricted. One of the more popular items are bump fire stocks. A new contender is the bi-directional trigger by Liberty Gun Works.