What do you call a E8 in the Marines?

A First Sergeant or a Master Sergeant. Which rank they actually are depends on their job. The rank insignias are different. They both have 3 chevrons and 3 rockers, but the First Sergeant has a diamond in the middle and the Master Sergeant has a pair of crossed rifles on the middle. It is improper to address either one of these ranks by shortening the rank names.

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