http://www.popularmechanics.com/military/a22451/history-gatling-gun/

In April 1861, Richard Jordan Gatling watched hundreds of Union soldiers march through Indianapolis, ready to ride the rails to the front. In the opposite direction came a never-ending stream of badly wounded men, to say nothing of those who lost their lives. Gatling set about inventing a 'labor-saving device for warfare' that he hoped would minimize the number of men needed to fight a war, and thus minimize the number of men exposed to its horrors.

Working throughout the summer of 1861, Gatling developed the invention that would carry his name though history: the Gatling gun. The Gatling is the most famous of late 19th century's manual machine guns. In an age of slow-loading rifles, the Gatling offered unprecedented firepower. Even today, a century and a half after its design, the gun retains its iconic status. So why does the world still know the name of gun that's been obsolete for a hundred years?

easy to build and legal in Hawaii.
It is classified as a semi auto rifle by the ATF.