Go in with the understanding that you will NEVER be top DPS in a proper party because you have tradeoffs to make for being ranged and for being able to extend the DPS of your party.

Good? Good. Read this guide: http://forum.square-enix.com/ffxiv/t...achinist-Guide

Currently the best guide to using MCH.

Also you can use Rapid Fire to more quickly switch into Gauss Barrel.