MP5

The Submachine Gun is a weapon primarily used by the HECU in Half-Life.

Overview
This SMG is the weapon of choice of the HECU military force (and the Military Police) dispatched to the Black Mesa Research Facility in Half-Life, and is one of the most effective and plentiful 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 difficult to avoid.

Strategy
The SMG's strength is greatly enhanced by the grenade launcher. A grenade plus a few rounds can quickly kill most enemies. Even if you don't have grenades, the SMG works well against headcrabs, Houndeyes, Vortigaunts and a small amount of HECU troops. Make sure that there's some cover near you so that you can run in, take a few shots, then hide before the enemy attacks.

Identity
While not actually named in game, the submachine gun weapon clearly resembles a Heckler & Koch MP5, with an attached M203 grenade launcher, and is referred to as the Navy SEAL's submachine gun (and real US Navy Seals indeed use H&K MP5s) in the Half-Life manual (i.e., it's the SEAL's version of the weapon, designed specifically for them, with a cam mechanism that allows for selective 3-round burst fire). Some players have ridiculed the MP5/M203 combination, as they have claimed that attaching such a grenade launcher to the submachine gun would be impossible, or at least impractical, but it is really possible to mount M203PI grenade launcher on some of the MP5 variants. Though not suppressed (ie. silenced, a wrong name) in game, the model is not a MP5-N (N=Navy) it is a MP5-SD (Sound-Dampened).

The game’s data files refer to the weapon as the 9mmAR, standing for ‘9mm Assault Rifle’. Weapons classified as 'assault rifles' which 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. 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 (that is probably a design oversight). This 9mm peculiarity has made the M4/M203 much more controversial than 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.

The M4 Carbine also appears in the Playstation 2 version. In that version, it has its own ammunition reserve.