A "Lightning Gun" is the standard and distinctive weapon used by Macbeth. It was named only once, in the episode "Pendragon", when King Arthur referred to as a "Lightning Weapon". The device has yet to be given a more "technical" name.
A "Lightning Gun" is an electromagnetic weapon with two electrodes that generate the charge. Macbeth uses it as his standard weapon, as did Banquo and Fleance when they were still under Macbeth's employ.
Griff appropriated a lightning gun when he fought Macbeth and his stooges, and later used the same gun when he fought against Xanatos's Iron Clan robot in London, remarking that he should get Macbeth to give out the guns to every member of his clan. ("Pendragon", "Rock & Roll")