USMC MOS QUESTION PLEASE READ!?!?

I am planning on enlisting once i finish school. I want to sign up for 0300 field. I am most interested in the 0311 mos but I am aware that the Marine Corps picks an mos for you that is in the 0300 field. I was wondering if this is true? Im not one of those guys who thinks thw 0311s just get some all the time I have a good idea what its really like but still want to do it. Is it possible that I sign up with the 0300 field with hopes I will get 0311 but end up an mortarman or atrillery man? 10 POINTS TO BEST ANSWER thank you!