I dont know the laws in Canada on buckshot but you could use a over under rifle shotgun. Put that low power scope on and you could use shotgun for close and rifle across lake. If Buckshots illegal then never mind.

I have a 7600 and used to use the high sights. They never felt comfortable looking thru scope without a rest but if I leaned against a tree or something for longer shots I didnt have a prob with it.

What about a magnified red dot scope. Ive never used one for long shots but my bud has one on his muzzleloader Ive shot a couple of times and it seemed to be ok.

I dont think youll get a bad caliber for what your doing with any they make in the 7600. I think the 7600 is alot faster for a second shot compared to a bolt.