Operation: Black Mesa is a total Source remake of Half Life: Opposing Force. Using the Source engine, the developers plan to recreate the Black Mesa research Facility as it was seen by Corporal Adrian Shephard of the H.E.C.U. Half-life: Opposing Force is still an amazing game, although it's not nearly as amazing as it was when it was released.
Weapon model that will alter the MP5 in case you prefer the HD-pack models. You will be able to choose from MP5 or M4A1 through the menu option.