1. Don't aim with Left Trigger, and just hit Right Trigger. Heavy-shot will shoot straight out from the side of your ship. You can't aim it. Just be really close to another ship when you use it. It does a ridiculous amount of damage. I love opening a fight by ramming an enemy, then double-tapping B to slow down and turn along side for Heavy Shot x2. It basically takes almost anything down to 1/2 health or less. Even those lvl 36 Man o' Wars.
2. You don't need mortars to use your telescope. Hold Right Bumper, and your telescope comes up. You can zoom it in/out to scout ships. It will tell you what they have right on the screen.
~ The only difference between genius and stupidity is that genius has its limits. ~
Thanks guys. After doing all missions and now redoing for the 100%, where I never used heavy shot before, and only after redoing a mission 4 times and missing the same heavy shot objective each time I realized it's not a glitch. Shooting without aiming probably sounded (and still sounds) as a bit stupid, but I can see now there are some nice advantages.