This Korean guy built his own M2 styled machine gun that shoots Nerf Darts. He was very meticulous in replicating the action of the M2. Of course there is no gun powder to make the action cycle so it is an electric system with fly wheels to propel the darts and manipulate the action.
It looks like most of the gun is 3D printed and the main housing is laser cut acrylic. He uses a magazine rather than a belt system. Which adds to the reliability of the system.