Sentinel Majors are upgraded versions of regular Sentinels equipped with more resilient golden armor, a blue Sentinel Beam which fires energy at twice the rate of the traditional red one, and protected by a light energy shield. They were on Installation 05 during the Battle of the Quarantine Zone.
The Sentinels are the defenders of the Installations and are controlled by the Monitor of the Installation. Their primary purpose is to contain small outbreaks of Flood; however, they can only manage Flood for a short time and in small quantity. Should an outbreak grow beyond the Sentinels' ability to control, Enforcers and Sentinel Majors are activated by the Monitor of the Installation to assist the Sentinels. It is also possible that there must be a massive outbreak, like the one seen on Installation 05, to give reason enough to construct these stronger versions.
In combat, their shields are less powerful than that of an Elite's, and the beam fired is blue instead of orange-red. It is also more powerful, but its battery drains faster as a result. You can pick up this beam and the normal beam as your primary and secondary weapons. One can tell the difference between the weapons simply by noting that the stronger of the two is gold in color, rather than silver.
Rarely, they can also be seen fixing pieces of Forerunner material, much like the Constructors do. However, this is extremely rare, and they cannot fix anything, even in groups, for massive-scale damage/repair.
Individually, their relatively low shielding (As compared to an Elite, for instance) is made up for by their maneuverability. As with Rangers, they can easily slice through the air, evading slow-moving projectiles (Such as Rockets) and coerce the combatant to concentrate on one at a time. The best weapon for them is the Covenant Carbine or the Plasma Rifle. First, the Covenant Carbine has a pretty strong punch and can penetrate the shielding in one or two shots. Then, in an additional two shots, the Major will go down. For the Plasma Rifle, you will rely upon its steady rate of fire to wear down the shields rather than take it out decisively. Shotguns can also be effective if the target is at close range. When facing these worthy opponents, be certain to have some cover within close proximity—as these Sentinels travel in groups, and can take your shielding out in seconds. Behind cover, use hit-and-run tactics to evade fire. Be sure to take one out at a time, as a wasted effort will be useless - the shielding will still be there, and you will have to take it out all over again. Once you have taken out the Sentinel, however, you can have the extremely powerful laser. This can take out the "legs" of an Enforcer in less than five seconds, whilst an ordinary Sentinel Beam takes over double the time. If your shields are down and you are out in the open, crouch (Or do something to evade fire quickly) and then activate your active camouflage. Use this time to duck behind cover and do not fire in this duration.