Hit barrel is same as Power Thrust that addition in formula not multiply.
ex. normal atk = 100%ATK , Round trip = 2500%ATK
Hit barrel state -> normal ATK = 260%ATK , Round Trip = 2660%ATK
Arm cannon and Cart cannon are skill that force element to cannonball that's happen in KRO , IRO and now thRO.
You can't use Elemental-weapon(ex. crimson weapon) , enchant scroll or endow weapon to change element of this skill.