From their website:
Master gunners are survivors of countless battles and have earned expertise that makes them some of the most valued artillerists in all of Cygnar. These men and women are versed in all the skills of the artilleryman from barrage firing patterns to the use of defilades. They are experts in the complex commands used to coordinate a battery of field guns. Where other armies make use of artillery as support, the master gunners orchestrate their batteries into precision instruments of war.