Any device you can depress or otherwise actuate to fire a gun is by default the trigger. A button, a switch, whatever you need to hit for rounds to go off, that's the trigger. More than one round per actuation of the trigger, and it is a machine gun.
If you want one easy motion with more than one round going off, and you want it cheaply and legally, then you want a Gatling type mechanism. Gatling type firearm systems are not considered machine guns, and are legal to own without any special paperwork in most states.