Well, you could download the "gamecore" folder from the
Modern Warfare Community Pack. It has a total of 18 weapons; one of them is Barret M82 .50 Caliber, with a custom firing sound. You will also need to download the "Props 3" folder if you want to have weapon-pickup entity. I don't think NPC's can hold the guns, and you will get a LOT more than just the gun (vehicles, characters, etc.), but if you really want it, it will probably be worth it.
Hope you find what you're looking for.
Captain Coder
One of the most humbling days in your life will be the day you discover you AREN'T the best at what you love to do.
- Me