Although they most likely will be all you need in many missions due to high ammo and power, autocannon usage is strongly discouraged in story missions because unlike AC4/FA you will lose money in seconds if you use them, however they do tear grunts to shreds and ACs as well. Used them on the story mission and although I beat it no problem, 90% of the profits were eaten away by them and I ended up with only a little over 900 Au profit.