I get so fed up with all this liberal vs conservative bull****.
Wanting to keep health hazards out of the environment has nothing to do political leanings - we all have to breath the same air, swim in the same water, eat food from the same sources, walk on the same beaches. It doesn't matter whether the pollutants get there from shooting bottles or throwing garbage overboard or industrial sources or... As sailors and users of the ocean we should be try to preserve it. And for those who think otherwise, let me come and take a sh*t on your doorstep and then ask you how you like someone polluting your environment.
Unfortunately human nature is to do what the hell we want, ignore the consequences and then come up with arguments to justify it. I doubt anything here is going to make any difference.
As far as the legality is concerned. It probably contravenes some treaty or international law or local law somewhere. I have no idea how it would be viewed but I can't imagine that the CG/ law enforcement wouldn't a least check you out if they become aware. I'd jump on ch16 if I heard gunshots from the vicinity of another boat.
As far as target practice is concerned, I think you should look for a safe, controlled location. Sail on the ocean - shoot at the range. I know that's not what you want to hear. Oh well!!