During the Napoleonic Wars, an enterprising Englishman, James Wilson, designed a 7 barrel rifle to enhance the firepower of sailors in hand to hand fighting while boarding ships at sea.  It was thought that the devastating blast of 7 barrels would clear groups of the enemy that were naturally packed together on the fighting decks of warships.  Named for the manufacturer, the Nock Volley gun proved to be a disappointment and a hindrance to the firer.  

picture of Nock Gun

The Nock Gun from the Rifle Shoppe

Being a flintlock weapon, ignition was always a problem, and at times not all the barrels would discharge successfully.  Loading under battle conditions was time consuming, and to ease this, the rifling was superceded by smoothbore barrels, as pinpoint accuracy wasn't the point of this gun anyway.  Lastly, the recoil was a lot worse than first thought by the inventor, and many a sailor sported a broken shoulder after serious use of the Nock gun.

The Nock Gun was made popular by the British TV drama about the Napoleonic Wars called Sharpe's Rifles, making it's appearance in this episode of Sharpe's Company.  You can buy a reproduction of the  Nock Gun from a couple of places, one being The Rifle Shoppe.  I got a kick out of this video from Youtube.

The dream of a many barrelled gun came around again during the Vietnam era with an armored vehicle called the Ontos, an armored platform sporting 6 106mm  recoiless rifles.  It's weakness was in loading, as the guns had to be reloaded outside of the hull, exposing members of the crew to enemy fire.  A number of them on the perimeter of a firebase were effective in fighting an enemy who had no armor or air support, but did have overwhelming numbers to swarm an American position, such as at Khe Sanh.  The 6 barrelled machines could fire beehive rounds, in effect being six giant shotguns per machine.  They had to back out of their position to a safe place to reload. 

picture of Ontos