There are 3 barrel duckfoot black powder pistols available from Dixie Gun Works. Never knew all barrels fired at once...so according to legal definitions, a black powder machine gun (i.e. more than one shot fired with a single pull of the trigger)!
From their page:
"All barrels fire at the same time. Instructions for assembly and firing included. Skill level 1. One piece grips. Brass frame. Barrel is .36 caliber percussion, steel,white round. Features brass triggerguard, steel barrel block, and single trigger. Manufactured by Classic Arms/USA. THIS PRODUCT CANNOT BE SHIPPED TO NEW JERSEY OR CANADA."