Halo

Alternate ending sequence:
Successfully complete the last level of the game in campaign mode under the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both.

Faster reloading:
To reload most weapons faster, press X and shortly after that press B to melee. Note: For the shot gun this will cause you to stop reloading all together.

Extra shotgun ammunition:
Take a shotgun that has 12 shots and 0 clips. After you shoot it a few times, you should be able to reload manually. You can do this about ten to twenty times.

 Faster Warthog weapon use:
This trick works best in multi-player mode. Drive the warthog in a straight line. When you see an enemy, jump out of the Warthog. Tap X repeatedly and sidestep to the right. If you are quick enough, you will exit the Warthog and jump on the LAAG gun without stopping, while the Warthog slows down. This also works well on ice, because you keep sliding.

 Defeating Covenants:
When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area.

There are places (especially in the first level) where a groups of Covenants are, waiting for you to barge in and start shooting. If you can, sneak to the aliens and throw a grenade into their midst. It is best when a plasma grenade sticks to a Grunt, because he will run behind the rest of his companions. One simple grenade can take out five Covenants.

 Defeating Flood types:
Do not try to take out any Flood types with a sniper rifle. It will simply go through their bodies without killing them. The weapon of choice is the shotgun. You can take them down with one shot. Remember, the closer they are, the faster they fall.
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On the small crawling Flood, use your assault rifle in short controlled bursts. Once one pops, it will also pop the ones nearby. Save shotgun and pistol ammunition for the working class Flood to drop them fast. It is also a good idea to hit them with your gun after they fall to make sure they are dead. On the fat Flood, use the shotgun to pop them, then clean up with rifle.

When you first meet the Flood, they will outnumber you in groups. For the large Flood, use a shotgun, rocket launcher, or most importantly, a fragmentation grenade. Do not use a plasma grenade because they will run towards you and kill you.

When you get attacked by the Flood, after you make them fall down the first time, shoot their head off while they are on the ground. They will not get back up.

 Defeating Flugs:
In the later levels of the game you will encounter new enemies called Flugs. These enemies will be small at first, but they will attack and control anything alive. Use a shotgun on these baby Flug as they tend to group in large numbers. By using the shotgun it will send a storm of bullets and kill most of them that are around. Use a shotgun on the possessed Flug, and it will kill most of them in one shot. This is made easier by the fact that most of them carry shotguns.

 Defeating Grunts:
When the Grunts are sleeping you can beat them to death with the butt of your gun without waking up any of the other Grunts. You can usually kill a four to seven of them before an alien spots you. They start sleeping in level 5, Assault on the Control Room. Just remember not to shoot or they will wake up.

 Defeating Hunters:
Normally Hunters are the hardest Covenant to kill, due to their powerful armor and shields. However, even the strongest armor has weak spots. If you look at their necks or backs, you can see an orange spot. Aim for it. One shot with any gun will kill them if you hit them there.

The best way to kill a hunter is to use a sniper rifle to zoom in on the orange spot.

If you are being chased by a Hunter or an alien with a sword, run backwards while facing the alien. Jump when they try to slash you. Throw a plasma grenade at the alien and it will stick to him. Turn around and run in a zig-zag pattern to make sure he does not catch you in the explosion.

If you are playing cooperative multi-player mode, have player two run in front of the Hunter until his back is turned to player one. While the Hunter is trying to turn player two into mush, have player one run up to the Hunter's back and melee attack the orange spot. The Hunter should die with one hit.

You can kill the Hunters in a head-on assault. Get in close and use the pistol or shotgun. Aim for the head holding down the trigger. Your first shot(s) will knock back the head, exposing the unarmored neck. One shot in the neck and they will die.

 Defeating Jackals:
Use a photon weapon to destroy a Jackal's shield quickly.

When you encounter a Jackal, run up and hit it with your weapon or get out a gun that uses bullets, not energy.

When you are a far way away from Jackals and have a sniper, they will sit in the defense position and leave very little uncovered. You will notice that on the right or left is a small notch in the shield. Aim for it and the Jackal population will be severely decreased.