A sidearm in Halo 3, much weaker than the Halo: Combat Evolved model with an even smaller clip. Due to the combined accuracy, power, and scope of the M6D pistol from the first Halo, the Halo 2 and 3 equivelents were significantly depowered. With the scope missing and smaller ammo capacity, this side arm is no longer the apt substitute for a mid-range weapon. It was thought, because of its chrome-type finish (similar to the M6D), that the pistol in this game was the returning scoped pistol.
The two advantages that this magnum has over its predecessor is its ability to be dual-wielded and slightly higher rate of fire. Despite this, the original SPARTAN side arm is still looked upon as superior.
Halo Reach Variant
The M6G, also known as the Magnum (not to be confused with the M6C Magnum
from Halo 2), is actually very similar to the M6D
from Halo 1--both in design and power. The Magnum has a fast rate-of-fire and can take down a Spartan player in 5 shots (assuming the last is a headshot), but is inaccurate when fired too quickly. It features a 2x scope and is most useful at a medium range, but will prove to be very effective at close range for players with an accurate hand.
Halo Reach does not have dual-wielding, so obviously the Magnum cannot be dual-wielded.