MP5

9mm Fully-Automatic Weapon (with Grenade Launcher)
 * Weapon category: 3
 * Range: Medium
 * Primary attack: Fully-automatic, 5 pts/bullet
 * Secondary attack: Launch single grenade, 100 pts if direct hit
 * Ammunition (primary): 9mm bullets
 * Ammunition (secondary): 40mm grenades
 * Capacity (primary): 50 rounds in magazine, 250 in ammo reserve
 * Capacity (secondary): 10 grenades

The firearm is the choice weapon of the HECU military force dispatched to Black Mesa, and one of the most effective weapons in the game. It uses the same ammunition reserve as the 9mm pistol. This automatic weapon compensates for its lack of accuracy with a rapid rate of fire and a 50 round magazine, and as if this were not formidable enough there is an M203 grenade launcher attached. While the primary attack’s spray of 9mm bullets can mow down most foes, when you find some 40mm grenades you can use the secondary attack to lob one into a group of enemies or send an explosive arcing through the air to dispatch an enemy hiding behind cover. Unlike hand grenades, these are not timed and will detonate on impact due to their centrifugal fuses, making them almost impossible to avoid.

In standard Half-Life this weapon resembles a Heckler and Koch MP5 with attached M203, and is referred to as the Navy SEAL's submachine gun in the Half-Life manual (i.e. it's the SEAL's version of the weapon, designed specifically for them). Some players have ridiculed the MP5/M203 weapon, as they have claimed that attaching such a grenade launcher to the submachine gun would be impossible, or at least impractical. This ‘unlikely’ weapon combination is used in real life by Malaysian soldiers of fortune, however, and it has also featured in action movies such as the 1999 film End of Days, starring Arnold Schwarzenegger.

The game’s data files refer to the weapon as the 9mmAR, standing for ‘9mm Assault Rifle’. Assault rifles that fire 9mm bullets exist in real life, but they are extremely rare. The 9mmAR name is a relic of Half-Life’s alpha development stage which featured soldiers armed with assault rifles rather than submachine guns. The Half-Life High Definition Pack replaces the MP5 submachine gun with such an assault rifle, while the attached M203 grenade launcher gains its own separate trigger. This replacement weapon has the appearance of a Colt M4 carbine (specifically an M4A1 and M203A1 grenade launcher) with a 5.56mm magazine, but it still functions the same as the original MP5 submachine gun and shares the 9mm pistol’s ammunition type. This 9mm peculiarity has made the M4/M203 as controversial as the MP5/M203 that preceded it, though it is generally accepted that the assault rifle looks more appropriate for a military force responding to an alien invasion.