M9 SMG

"OK. Time to clear up some misconceptions about the M9 Sidearm. It is not a high-speed low-drag tool for ninja operators. It's a gun for REMFs who may stumble into a firefight one day, but can't be arsed to haul around a full rifle. The UNSC procured them so that when the Covenant blitz through the front lines and kill a truck driver, the generals can keep a straight face when they tell the next of kin that he went down fighting."

- Comment by the user WOWizard12 on the unofficial MarineNet Message Board.

The M9 Sidearm, formerly known as the DT-92 and commonly called the Truck Gun or the Blitz Brick, is a firearm issued by the UNSC Army to vehicle crews and non-combat personnel. Its distinguishing features are a high capacity magazine and dead-certain reliability, even after having been left in an exposed cab for a decade.

History
Although its design lineage traces back to sidearms issued to EVA personnel in the Inner Colony Wars, the DT-92 submachine gun was originally designed by Dahl-Tech in 2492 in response to a CAA tender for a compact submachine gun. The design was iterated and improved somewhat over the following decades, but key design shortcomings persisted when it was procured by the UNSC Army in 2527. Nevertheless, it was cheap, costing only a quarter of the price of an M6C magnum.

In adopting the M9, the Army faced a choice between adopting a new sidearm or procuring more M7 submachine gun. The deciding factor came down to cost and reliability. The M7 submachine gun is as reliable as any other gun, so long as it is kept by the armory and fed ammunition in good condition. Caseless ammunition is more susceptible to environmental degradation. The Army was procuring a truck gun, where ninety nine in a hundred such guns would only ever be fired for yearly qualifications. For this reason, the M7 was dropped from consideration. The DT-92 would make it through trials and eventually be adopted as the M9 Sidearm.

The M9 submachine gun was in mass-production throughout most of the Human-Covenant War, with millions being fabricated despite early problems with meeting the Army's quota demands. However, the loss of key industrial centers made it more and more difficult to supply the UNSC's diverse range of firearms and calibers. Under pressure to at least standardize their ammunition with the other branches, the Army began slowly supplementing orders for M9s with new M6J/C carbines throughout the late-2540s, with the goal to eventually replace them.

Design
The gun is chambered in 7.62x35mm SOCOM, a straight-walled cartridge of moderate armor penetration capability. The cartridges are fed from a 50-round magazine that sits under the barrel of the gun. The gun must unfold to remove the old magazine and insert a new one. While this makes reloading a cumbersome process, the Army did not expect such firefights to last long enough to require a reload. When reloading, the M9 Sidearm hinges open at the front. The barrel, bolt, and recoil assembly is in the upper half of the gun, while the trigger assembly is in the lower half of the gun. This introduces the key advantage and disadvantage of the M9. In addition to the slow reloads, the upper half of the gun 'rattles' on the front hinge. While the sights are fixed to the barrel, the barrel's connection with the pistol grip is loose enough that the M9's paper accuracy (though not necessarily the practical accuracy) is lower than the M6 series or even the M7 submachine gun. At the same time, the firm seal between the magazine and the gun keeps dirt and mud out of the action, ensuring that the gun will fire even after being neglected for years.