One of many weapons produced by Durandal and his S'pht allies, the .44 Magnum Mega Class A1 was the successor to the .45 Magnum Mega Class. This weapon was a mainstay of the BOBs that fought to free the S'pht homeworld Lh'owon from Pfhor occupation.
Manufactured aboard Boomer, these weapons were functionally identical to their predecessors. The Mega Class A1 is a semi automatic, scoped, vacuum enabled pistol with an eight round magazine. However, the caliber is only .44 caliber compared to its predecessors .45 caliber. The weapon was also stated to have had a more ethical construction process. 
The magnum is the player's starting weapon and serves as the workhorse, just as it did in Marathon. Just like Marathon, the weapon is capable of being dual wielded and is by far the most consistent long range weapon the player has.
Unlike the .45 Magnum, which appears to be a revolver, the .44 appears to be a semi-automatic handgun.