M90 Close Assault Weapon System is a pump-action 8 gauge shotgun. (Translation: N00B PWNZ0R XTR3M3) Since mostly campers and noobs use them, they are usually found in hard-to-find places, so the noobs can fight over them. With the advent of Halo 3, however, it has become more common for shotgun user to come out into the open, in order to fire rapidly at enemy Assault Rifle users. Warning: this tactic is not advised, unless said AR users try to kamikaze.
Recently, there've been reports of 1337 players who use shotties in the open and rack up killionare exterminations on an hourly basis. Whoever these people are, they must be gods of the new universe.
Shotgun Mk 1Edit
The original Shotgun Mk 1 from Halo 1 was the ultimate holy weapon. It could kill tons of crap in one shot (including those damn terrorists later found to be the Taliban) and had a kickass reload rate that made the Covenent ships turn on each other just to steal a huge armory filled with nothing but shottys.  It comes in only one variety, and that is badass, and it makes people's heads asplode. It is good for making squid heads crap themselves and pwning Guys with AIDS. It is also extremely effective at killing Grif.
Shotgun Mk 2Edit
In Halo 2, the idiots over at the UNSC decided to replace all standard Mk 1 Shotguns with the new Shotgun Mk 2. The main upgrades were new safety features designed to reduce friendly fire casualties. Unfortunately, these same safety upgrades also served to severely reduce unfriendly fire casualties against the AIDS, not to mention less reload speed.
If you are more than 30 centimetres away from the target, you barely do any damage, which catches your target's attention, making them turn around and stab you in the face. Despite this, it was still able to kill Grif.
Shotgun Mk 3Edit
Finally, in Halo 3, the UNSC decided to compromise with the Marines and give them back some of the features of the old Mk 1. The new shotgun was nearly as powerful as the Mk 1, but only against certain alien species; it was utterly useless against the rest of them. At least they gave it a kickass pump, but at the cost of HALF THE FUCKING AMMO!!
Shotgun Mk 4Edit
Upon defeating the Convention and creating a cure for AIDS, Sergeant Johnson and Master Chief had no one to kill anymore, so they declared war on Elites. But the Elites, having shields and superior close-range weapons, owned their asses. So, the UNSC created a triple-barrelled version of the shotgun. The Elites got fucked, so they left to go back to their frozen-ass planet. The humans then used the shotgun to blow people's heads off, so it was permanently discontinued. They are available on EBay for $17, though.
Shotgun Mk 1337Edit
In R33CH, Bungie decided not to be a bunch of noobs, and joined up with the guys from CoD and created a new shotgun that could instantly kill anything in close range, even leet dudes like Hunters or shit like that, and only 1337 people with m4d Evade sk1llz or N00BZ with Arm3r L0k could live through it, though the latter died 3 seconds later.
For many times now people have been using Shotguns. Like when you went to the Library with the Eye ball guy you pissed everyone off because you were loud..then stupid AIDS people came with shotguns. They may have not known how to use it but they pissed of Master Chief ALOT. It's history has gone far all the way to Xbox Live where noobs don't know what they're doing and they probably kill their own doodz.
For all intensive purposes, although a mere 20-gauge will drop a freaky mutant ostrich thing from Reach from 100 yards, these 8 gauge magnums can't do jack-shit beyond a few meters.(so what if im mixing systems?)
Usage of the shotgun is quite simple.
- Load the shotgun.
- Pump the shotgun
- Point in the general direction of the thing that pisses you off.
- Stand back, press trigger, and watch guts and metal shit fly everywhere.
- Repeat ad infinitum until the glowing moron is dead.
- Profit! Note: If done improperly, you will suffer instantaneous death by way of a huge-ass Lazor to the face.
- Respawn, rinse and repeat. Yell "Boom, you are dead!" for flavoring and reload for added texture. Remember to not run out of ammo nor get hit by another person with the Shotgun.