M9 fragmentation grenade
From Halopedia, the Halo wiki
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The UNSC-issued M9 HE-DP Grenade functions in a manner very similar to contemporary hand grenades. A blast bursts the grenade's shell open and sends countless small metal fragments flying at extreme speed, causing damage to energy shields, armor, and flesh in around ten meters (30ft) radius. Four may be carried at a time. An "infinite grenades" option is available in multiplayer on a single Xbox, as well as in all multiplayer Halo PC games.
Technicalities
HE-DP stands for High-Explosive Dual-Purpose. The first phrase indicates the grenade uses a high-explosive substance to propel it's shrapnel when the grenade explodes, while the second mean it can be used against both infantry and vehicles (hence "dual-purpose").
The grenade has a hard metal casing that is smooth in Halo 1, but has a gridded surface in Halo 2 and Halo 3. This griding is most likely used to provide a better grip for the thrower, and contrary to popular belief, would not influence the fragmentation pattern of it's shrapnel. Instead, the grenade's outer metal casing, presumably made of steel or another metal (such as tungsten) or an alloy, would be blown to pieces when the HE core inside detonates.
AP FRAG can be viewed on the Halo 1 M9's olive casing. AP most likely stands for "armor-piercing", indicating the shrapnel of the grenade is capable of piercing solid armor. In both Halo games, and most likely in Halo 3, the grenade must first hit the ground before exploding.
In Halo 1, the grenade had to come to a rest, but in Halo 2, it would bounce and explode mid-air a moment after bouncing. This was promoted as a safety feature, ensuring the grenade would only explode after striking a surface after being thrown, and not in the user's hand.
This also suggests the lack of a "spoon", or handle, on the Halo 2 and 3 M9 HE-DP grenades. The spoon is a device that is gripped while the grenade is being primed, and ensures it will not explode until after the spoon is released, as it starts the fuse of a grenade as it flies through the air after it is thrown. In Halo 1, the spoon would have negated the need for this Halo 2 safety feature.
It is unknown how the M9 is first primed, but references to pulling a pin on the grenade is mentioned in the Halo novels several times, even though no pin is visible in gameplay. However, pulling a pin would require two hands, but each version of the grenade has a button on it's body. This would most likely be pushed to prime the M9 before tossing it. The pin reference is most likely an inaccuracy on the author's part in the novels.
Advantages
This grenade can be bounced around corners and travels farther than the Plasma grenade. It is also harder to see and has a shorter fuse, therefore making it harder to avoid and more useful for killing enemies on foot. It also seems to have a larger blast radius than a plasma grenade in Halo 1.
Disadvantages
Unlike the Plasma grenade, this grenade doesn't stick to objects, making it harder to hit vehicles. It also doesn't kill enemies like Brutes, Elites (although it will destroy its energy shielding), or Hunters instantly. Since it is harder to see, running over one by accident can happen more easily than with a luminescent plasma grenade. If tossed carelessly by the player and lands near a Marine, the marine will stare at the grenade and yell an obscenity at the player such as: "Watch where you throw those things!"
Legendary Mode Tactics
Use the M9 HE-DP Grenade when Elites and Grunts are in large or small groups as this will kill many Grunts, damage Elites shields and sometimes Grunts will be stunned for a short period of time making them quick work with the M6C Magnum, or Battle Rifle. (Note: This tactic works extremely well on Cairo Station when enemies come out of boarding ships.)
Multiplayer Tactics
A multipurpose weapon, the fragmentation grenade can be used in corridors to flush out occupants. If lobbed under a vehicle, the explosion will either kill or severely injure the occupants. If timed correctly, grenades can be planted in the path of an oncoming vehicle. Most drivers will not be able to react quickly enough, and will most likely be caught in the blast.
Effective Against
Anything land based except for heavy tanks such as the Scorpion Tank and the Wraith Tank.
