M6C Magnum

In an attempt to balance the abuse of the popular scoped pistol from the first Halo, the M6D, Bungie responded by creating two seperate weapons for its inevitable sequel.  The Battle Rifle came to inherit the scoped function of the pistol while a new sidearm was developed in the form of the magnum.  It became obvious to gamers that this gun was a poor substitute with a much lower accuracy and ranged power rating, but was still somewhat effective in close combat and had the ability to be dual-wielded. 
The poor reception prompted the development of a "happy medium" between the two guns, leading to the creation of the M6G for Halo 3.  Although it was only useable in Halo 2, the M6C is featured being wielded by UNSC marines in Halo Wars cinematics, and appears to still be in use by the Marines in a sidearm capacity. 
 

SOCOM Variant

Halo 3: ODST presents a SOCOM variant of this weapon, often referred to as the "Automag".  Although having more in common with the M6D, containing both a scope and a seemingly higher stopping power than its Halo 2 iteration, its increased rate of fire is indicative to the M6C.  The weapon is also fitted with a supressor of sorts, making it an invaluable sidearm for the ODST who wants to pick-off Grunts while maintaining a relative level of stealth.