I like this. It's definitely a move that has been asked for by many, and if they end up following through with it, it'll be a wonderful addition.
I do say that they need to be balanced differently...
For the MKII Gatling, I'd rather it just be a twinned Lewis gun, similar to what was used on WWI fighters. It would appeal to a lot of people, and it would kiiind of be lore/ weapon mechanic friendly, as a rotary barrel IS designed to minimize recoil so as to make a weapon more accurate at range.
For the Hwacha MKII, I love the idea for that design, but only one head. Cut the ammunition amount and make it much more accurate, longer range and more effective at range, even without Heavy clip. The Hwacha is overused too often, and this would cut the monotony of it, and offer up a new way of doing things.
Flak, we need a howitzer. I will say point-blank-period, howitzer. Flak is built to be more noob friendly, and they could nerf it's damage a little more in order to give new flak even more speed and less drop, meanwhile, the Howitzer would have everything Old Flak had, but maybe with a faster reload than new flak.
And yes, torpedoes. Torpedoes would be fantastic, though I seriously doubt it would be a light... A torpedo system would have to be heavy.