Halo 3 Sergeant rating problem?

[Father Dougal McGuire]

Hi, I've just reached 20EXP in Halo 3 and achieved the Spartan Insignia on my service record. But I haven't unlocked the achievement or unlocked the Mark V helmet. Does anyone know why?
I don't know about the campaign but 20EXP is Sergeant. Look it up, Cheesecake.

 

[curely cuz]

1st 20xp is like corpral and thats rubbish yopu need to complete campain before you ge the helmet

 

[Mysterious Answerer]

I think that used to be the old way to get the achievement now i think you need to get 20EXP in a playlist like lone wolves and to get the Mark V helmet its probably same as the achievement reach 20EXP in a playlist and to get the sparten officer you need to get 50EXP in a playlist the best way to get to 50EXP in a playlist fast is to win in a double exp weekend that way you get 2 instead of 1 or play Lone wolves you still exp even if your 1st 2nd or 3rd.
You must have the achievement to get the Mark V Helmet.

Good luck trying to get the achievement.