The gun he used was a Sig Sauer MXC. In its original form, it IS a machine gun (Machine Gun: noun 1. an automatic gun that fires bullets in rapid succession for as long as the trigger is pressed.) as it can fire fully auto ("...even with a massive silencer attached, shooting in full-auto, the MCX is extremely controllable."). He may have been using a semi-automatic civilian version, but it was still a variant of a machine gun, and as pointed out above there are ways to modify it to get around the restrictions, and machine gun is also defined as "any weapon convertible to such a state", so at best it is a semantics argument, not a substantive one.