MP5

The MP5 is the fourth weapon the player, Gordon Freeman, receives in Black Mesa, being a heavy sub-machine gun commonly wielded by Hazardous Environment Combat Unit grunts and first seen in We've Got Hostiles. MP5s share the 9mm ammo pool with the Glock, and all come with an underslung grenade launcher.

Description
The MP5 is first encountered in the chapter We've got hostiles. It must either be taken from a dead HECU trooper, or looted from a nearby ammunition crate.

In-game
The MP5 is a full-auto firearm, effective up to medium range, and if you can learn to control its recoil, it'll be one of your best friends. If not, there's always the shotgun.

Primary fire is a very respectable 10 shots a second, each bullet dealing 8 damage. Fire in short bursts for higher efficiency, at least until you learn to control the MP5's recoil. Ammunition for the MP5 is found in huge quantities wherever there are HECU troopers, so unless you use only the MP5, ammo management shouldn't be a problem in the base game.

Secondary fire instantly engages the MP5's underslung grenade launcher. The grenade is launched in a low arc, so to hit targets at medium range, you're gonna have to aim up a bit before firing. The 40mm grenades have a moderate blast area, and adequate damage (115) to take out roughly 3 humans or small xen aliens in a single blast. There's no downtime after firing a grenade; one can squeeze out all three in very rapid succession. 40mm grenades are typically found in lots of 3, and since that's the max capacity, there's really no reason to try and hoard them.