As the treacherous Atlantean King sought to enlist the aide of the pirate whom he'd contacted in order to secure a surface based vessel, Orm would present David with a colorful arsenal of next generation mariner weaponry as a method of enacting revenge for the death of his father.
Using his expertise in engineering, Black Manta would study the innermost workings of the rifle. How it worked, power source, the firing mechanism; all of which he would rework into a wearable piece of diving equipment that also functioned as a powerful energy cannon. One which Manta tested to his hearts content while pursuing Arthur in Venice; Italy, able to badly sear and burn the Aquaman despite his incredible resilience.
Like typical Atlantean weaponry, this pulse bazooka took the methodology of typical undersea plasma artillery; which is the conversion of water to vapor and then the ionizing said gaseous substances. Into high yield ionic gasses which are hurled over great distances with incredible, deadly force from two firing ports.