CH252 helmet
From Halopedia, the Halo wiki
The CH252 Helmet is the standard issue UNSC Marine helmet and an integral component of the Marine BDU. What appears to be an earlier version of the helmet is used by the Marines stationed at the UNSC Pillar of Autumn, and an even earlier verison is seen used by the Marines stationed on the UNSC Spirit of Fire.
There are several versions of the helmet seen worn by different different field specialists. The standard helmet weighs about 3.3 pounds, according to crates found in the game, and is painted olive to match the standard issue UNSCMC body armor. This version can be fitted with ballistic goggles, in either orange or black, although the latter appears to be most most commonly found on sergeants or squad leaders. The ballistic goggles are more common if the Halo 3 Thunderstorm Skull is turned on. In Halo 2 and Halo 3, you can shoot off their helmets. They will not pick them back up.
The standard issue helmet fitted for the ODST body suit sports a slightly shorter brim, with a full covering reflective faceplate with HUD supporting abilities. The ODST version is made out of titanium armor, and features a matte black trim to match the body suit and is sealable to be used in hard vacuum. It is unknown if the standard UNSCMC helmet and pilot helmet are made out of titanium as well, or polymer, although considering the weight, the latter is more likely.
The Marine pilot helmet is another version of this type of helmet. This was most likely manufactured around the end of 2552. It is not seen until Halo 3. This helmet is almost a replica of the ODST helmet. It features HUD support and weighs about four pounds. It features a olive trim like its standard Marine counterpart, and has a black or clear visor. Each also bears a wing symbol, likely denoting their particular specialty. By 2553, this helmet became more common.
