a question about the American Army

I am writing a story that involves a group of soldiers during the first Gulf War, and have a question about how soldiers refer to each other. For example, according to what I found online, there is a rank called "specialist" which is above Private and below Corporal. How would say a Sargent call for such a person? How would someone distinguish between a Sargent and say a Master Sargent?

Any advice appreciated.