Manufactured by weapons systems company Heckler & Koch, the MP5 is the most popular submachinegun in the world. Introduced in the mid-1960s, it has been adopted by many military, counter-terrorist and law enforcement groups around the world. While over 120 variants of the firearm are available, the main types include the 9mm MP5A3 with optional 0-1-2 trigger group and tactical forearm light, the 9mm semiautomatic-only MP5SF, the 9mm MP5N (MP5 Navy) with ambidextrous trigger group and threaded barrel developed for the Navy SEALs, the compact 9mm MP5K-PDW and the 9mm/10mm MP5/10SD with an integrated silencer that doesn't require subsonic ammunition.
Ashley Pomeroy was indeed correct in stating that one of the weapons in Half-Life was indeed based on the MP5, however that is the original version. The High Definition Pack included with the Blue Shift expansion pack replaces all the weapons in Half-Life with newer, higher-quality models. For instance, the Glock pistol is replaced with something resembling the Beretta 92f pistol, whereas the MP5 is replaced with something closely resembling the Colt M16 rifle with M203 grenade launcher, as Velocity said.