It could be just that, legal but annoying gunfire :-/
The Full Auto part, if true might be more the issue.
Its legal too possess full auto weapons with the right FFL, but I'm not sure if you can fire them just anyplace?

DB, was on too something. Although I'm not sure I would want too confront people about a campfire I had issue with, when they also were firing a lot of rounds, legal or not.
I would probably paddle over EARLY next morning and act like I was just cruising by and check out the situation. IF anyone was up, I'd act naive and say something like " Did you guys hear all those FIREWORKS last night?" I figure if their morons, they couldn't resist telling someone about the firepower they had with them, and if they were in the right, they would tell you as well.
 Then I would determine whether they were just unaware of the potential problems, fires and gunfire can cause, or just didn't give a damn?
From that I'd either try too "enlighten them", or paddle on, noting numbers of party/ canoe registrations, and any physical descriptions and report that too the rangers/police when I got out.
I might even ask too take a few pictures of "campsite" for personal reference.
I'm sure there is more too this story, time will tell?