Nope. Didn’t know that. That’s gotta add weight.
Sure does. But it also prevents gas tube burn-through, another M16 family problem after a half-dozen Beta-C mags or Chinese LXX drums are cranked through on ratty-tat.
Add mass to the gas tube, and some cooling fins. That’d be the easiest patch. Next, try better materials. Tungsten perhaps. A water jacket sounds like a crazy idea to me.