Happiness Is A Belt Fed Weapon T-Shirt US Marines Machine Gunner MOS 0331 T-Shirt. Digitally Printed on soft superior 100% Cotton, Triblend, 50/50 premium Soft Style Quality T-shirts. Not Slim Fit Size XXL is Heavy Cotton. Marine Corps Infantry Machine Gunner handles large machine guns in direct combat. heavy machine gunners specifically handle the 7.62mm medium machine gun, the 50 caliber and 40mm heavy machine gun, plus their support vehicles. the design consists of a loaded machine gun with the words happiness is a belt fed weapon.