Projectile modifier spells that add an arc of material between multiple projectiles. Arc spells require a minimum of two projectiles in order to function
Gunpowder Arc is a modifier that causes a stream of exploding gunpowder to flow between projectiles. Unless the spell naturally produces more than one projectile per cast, like Triplicate Bolt, it must be used with a multicast modifier to work.
As a fairly dangerous modifier, Explosive and fire Immunity Perks are recommended.