From Halopedia, the Halo wiki
The M90 shotgun fires fifteen pellets per shot. The combined force of each relatively weak pellet does a significant amount of damage against almost any type of infantry, and the wide spread of the pellets in certain situations can inflict shrapnel damage to multiple infantry. The M90 shotgun's powerful 8 gauge round can punch through armor and energy shields easily, making the M90 a great weapon for taking on heavily armored infantry and lightly armored vehicles. The M90's top-loading system allows a user to reload the shotgun with speed and ease for quick firing.
While highly effective at close range, the M90, like all shotguns firing buckshot, is rendered useless at long range, and should therefore be used in conjunction with a medium or long range weapon. The shotgun's pattern is extremely loose, and fans out quickly; thus, at farther ranges, only one or two pellets out of the 15 might hit. The M90, being a pump-action shotgun, must be cycled after every shot before it can be fired again; the M90 must also be reloaded shell by shell and does not use any type of external magazine. This can make reloading time-consuming when in combat situations, but it is useful to be able to "slam-fire"; the ability of the user to interrupt the reloading and shoot after you load a round. In close-quarter situations, the wide spread of the shell can potentially harm friendly infantry if the shotgun is fired at both the enemy and friendly troops. In real life, the ejection port being on top is itself a disadvantage, as the inside is partially exposed, meaning it is vulnerable to rain, dirt, and other elements. This will allow the magazine tube to become damp and/or dirty, and can cause malfunctions and stoppage.
Differences among games
Halo: Combat Evolved
The M90's incarnation in Halo: Combat Evolved is arguably the strongest version. It can hold 12 rounds, and the player can carry up to 60 more rounds for a total of 72 rounds. It has the longest and most realistic effective range of all the three models, and its fifteen tungsten pellets could kill, weaken or injure targets at short to medium ranges. Although it is more likely to kill a closer ranged enemy as the shotgun is weak on further distanced enemies, the shotgun could also kill nearly any infantry target at point blank range. It even has the capabilities to take on lightly armored vehicles, such as the Ghost effectively. Players could hold the trigger button down for continuous fire. To melee, the player swings the shotgun up and brings the butt of the weapon on someone's head/front. If the player fires the entire magazine or if the player picks up a Shotgun without a full magazine and fires it empty, the character model will cycle the shotgun's slide after reloading, although most players usually opted to reload after every shot. It reloads more quickly than in Halo 2.
In Halo 2, the M90 changed significantly from its predecessor. The only aesthetic difference is that the iron sights are green instead of blue, and the overall length of the barrel is shorter. The melee attack is much faster than in the original game, and the weapon is slung horizontally instead. Its performance is also drastically different as it can not kill targets at more than a few feet away, though a quick shoot-melee combo (the infamous "shotgun bang-whack") would usually kill an enemy very quickly. The player can only carry 36 spare rounds for a total of 48 rounds. Unlike the previous version, it has little power when used against vehicles, and like before, with the range decreased, its effectiveness against the Flood was reduced slightly. The weapon's action does not cycle after reloading, unlike the previous iteration of the M90.
Halo 3 and Halo 3: ODST
The M90A is used in Halo 3 and Halo 3: ODST. There are a variety of graphical and gameplay changes from the previous models. The blue iron-sights have been reintroduced, it has a longer barrel and the flashlight is now mounted on the side. Unlike the older models which had a 12-round magazine, the M90A only has a 6-round magazine. The player can carry 30 spare rounds for a total of 36 rounds. To make up for this, its effective range is far greater than that of the Halo 2 incarnation, as it can kill targets at two or even three times the range, depending on their health. The slide is always cycled after reloading, despite the fact that this would eject a round if one were already chambered. Like in Halo: Combat Evolved, the M90A is once again a powerful weapon for use against the Flood because of its increase in range and power, though the M90A may still require a player to shoot and melee a Flood target, depending on the type.