Que? You choose which guns to use with the camera view. Forward is chainshot, side aim is regular round shot, side unaimed is a power scatter shot, rear is fire barrels, and L1 is Mortars. Once you start fighting big ships, there is very definitely strategy involved. (also, ramming)
You can't just slide up next to a Man o War and exchange cannon shots, for example. I have everything but elite upgrades for my Jackdaw, and I've gone after one or two Legendary ships and had a hard time outgunning them. You really do have to play around with strategy.