Volley gun
The Ribauldequin was a medieval version of the volley gun. It had its barrels set up in parallel. This early version was first employed during the Hundred Years' War by the army of Edward III of England, in 1339. Later on, the late Swiss army employed it.
Volley fire
Frederick Lewis Taylor claims that a kneeling volley fire may have been employed by Prospero Colonna's arquebusiers as early as the Battle of Bicocca (1522). However this has been called into question by Tonio Andrade who believes this is an over interpretation as well as mis-citation of a passage by Charles Oman suggesting that the Spanish arquebusiers kneeled to reload, when in fact Oman never made such a claim. European gunners might have implemented the volley fire to some extent since at least 1579 when the Englishman Thomas Digges suggested that musketeers should, "after the old Romane manner make three or four several fronts, with convenient spaces for the first to retire and unite himselfe with the second, and both these if occasion so require, with the third; the shot [musketeers] having their convenient lanes continually during the fight to discharge their peces." The Spanish too displayed some awareness of the volley technique and described it in a military manual dating to 1586: "Start with three files of five soldiers each, separated one from the other by fifteen paces, and they should comport themselves not with fury but with calm skillfulness [con reposo diestramente] such that when the first file has finished shooting they make space for the next (which is coming up to shoot) without turning face, countermarching [contrapassando] to the left but showing the enemy only the side of their bodies, which is the narrowest of the body, and [taking their place at the rear] about one to three steps behind, with five or six pellets in their mouths, and two lighted matchlock fuses … and they load [their pieces] promptly … and return to shoot when it's their turn again."