Actually you unlock MC's helmet at rank general or inheritor which is really high and its called the mljonir mk V. there's another one in which you can get at Lt colonel but its just mk. V not mljonir mk V and you can see the difference if you saw them together so hope this answer helped
Lt. Colonel
you have to work up enough credits and have to be the right rank to buy it
Do a bulltrue in matchmaking to unlock the avatar award.
Almost at the end of the game you will see the pelican that Capt Keys is in press right on the right stick and it will move over and you can see Master Chief in a criotube.
He is wearing the Commando helmet that you unlock by ranking up.
Commander
5h1t
Simple: you must be rank commander or higher. Emiles helmet is a helmet of its own.
Inheritor, the last rank in the game. The last helmet, the Haunted Helmet, is the Pilot Helmet with a skull on it. To unlock it, you must reach the rank Inheritor, and own every BASE helmet in the game.
You have to be an warrant officer in halo reach to unlock the helmet you also have to go to halowaypoint and unlock the armor.
only in halo reach you can to that
You need to be a brigadier and have 300,000 credits