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Sierra, in the NATO Phonetic Alphabet, stands for the letter "S." Spartan-117 is the Master Chief's callsign. Spartan obviously begins with "S." Thus, "Sierra-117" equals Spartan-117.

Actually, "Sierra" doesn't stand for Spartan. Otherwise, B312 would be S312. Chief is S117.

It does stand for spartan, B312 becomes SB312. Master Cheif Petty Officer John -117 is S117.

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