The .44 Magnum Mega Class (or .45 Magnum Mega Class in Marathon) is the starting weapon for the player and is also one of the most accurate in-game.
The Magnum is a default starting weapon of the player in all three Marathon games. Upon starting a new game, the player is given one .44 Magnum Mega Class and three clips.
The pistol is very useful in the early stages of the game as it is the only ranged weapon the player has. But once the player has acquired the Zeus Class Fusion Pistol or the MA-75 Assault Rifle/Grenade Launcher the Magnum is generally not used, unless the player is low on ammunition for their other weapons.
Similar to the WSTE-M5 Combat Shotguns in Marathon 2: Durandal, the Magnum can be dual-wielded. This allows the Magnum to regain its usefulness in the later stages of the game, as dual-wielding essentially doubles this weapons damage output.
Even though the pistol has a scope there is no way to aim-down-sights, The scope may represent in-game the reason the Magnum is one of the most accurate weapons.
Vacuum and Underwater
Both versions of the Magnum can be fired in a vacuum, but will not work underwater.
"Standard issue for all security officers and active duty field personnel. Utilizes 8-shot clips of .45 caliber high velocity, vacuum enabled teflon rounds (teflon maintains stable flight for accuracy). Titanium construction results in a sturdy, reliable, light weight firearm."
- from Marathon manual
"The new standard issue sidearm for all field personnel doesn't look much like your old .45 MMC, does it? That is because it has been built for a single purpose, by a dedicated people and not by underpaid laborers toiling for a thinly-veiled government-owned arms manufacturing machine (oiled with the blood of the underclasses, by the way) which exists solely to line the pockets of the greedy bureaucrats who run the military/industrial complex."
- from Marathon 2 manual