I can see the idea of a 'Gun Martial Art' existing. Lets take firing a rifle from a military point of view. In the Marines we fired from the standing, kneeling (which has three versions), sitting, and prone. At close range there's a differnit standing, and two more versions of the kneeling. I geuss you could call those stances.
At long range, we fire Tri-Fires, Slow-Fire, and Rapid-Fire. Close range there's Control Pairs, Hammer Pairs, Presentation Drills, Failure Drills, Box Drills, Rapid Re-Loads, and so on. I geuss those would be the techniques and drills. When you get to MCT you start to do more close range fire, and unknown distance, and firing on the move. Those would more advanced techniques I geuss.
When you do the qual for each table (which is a set combination of positions, ranges, and drills) that could be called a kata. Weapons maintaince could be looked as warm up/stretching/cool down. Then you could go into other weapons (the SAW, M240 Bravo, AT4, M203, M67, various other machine guns, rocket launchers, handguns, rifles, and shotguns) those would be like dan-level material.
So, a "Gun Martial Art" is possible, but doesn't really have much use outside of the military. Oh, and grappling is a bayonet (you either swear by it, or think it's freakin useless)