From Halopedia, the Halo wiki
A Turret Case is a UNSC Storage Device for a 7.62mm Gas Operated Linkless AIE-486H Heavy Machine Gun, and its accompanying tripod.
It is one of the many objects available in Forge. It comes in two varieties: the full case, and the lid and the case, without the lid.
They are mainly used to block door ways or build bases. The cost of the case varies from map to map. The lid costs three Forge points, and the open case costs four. The case is on all maps apart from Snowbound, The Pit and High Ground. The lid and open case are only available on High Ground.