I plan on enlisting in the USMC but i was wondering on how you end up with your weapon?

like i sed i plan on enlistin in the MARINES but i want to know how you ened up with your weapon? I specifcaly want to be a heavy machine gunner in an infantry unit. I dont mind on getting an m16 or an m4 but i would prefer the big guns so somone answer that knows what there talkin about.....