From Halopedia, the Halo wiki
Combat barriers are placed and situated all around strategic locations as defensive mechanisms against enemy weapons flanking fire. Combat barriers are used to protect soldiers of all types that are located in military bases, dug-in fortifications, and other facilities (such as ships). While many models of combat barrier offer decent protection from plasma and projectile weapons, they do not protect well against the concussive force of an explosion, though they do a good job of absorbing shrapnel and flying debris.
The M72 Mobile Barrier is a principal type of combat barrier used by the UNSC, encompassing the M72-S (small) and M72-L (large) Mobile Barriers. These barriers work by manipulating lines of sight; they have a small slit, opening or corner for someone inside to shoot out of and have full line of fire on his enemies, but the enemies must hit a very small target. Some combat barriers however, lack these slits - combatants using them for protection must stand up or creep out from the side to fire on an enemy, exposing themselves in the process. Beside the more common barriers made of metal, the UNSC also uses large concrete barriers.
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