You must first complete the gun shop owner's first mission in Act 2.
Like all his previous missions, you just head for an arms convoy that will drive around and around the same circular route, continuously and relentlessly, for no apparent reason until you get there and blow it up.
When you return to the shop, for 35 rough diamonds she's yours. It's a one-hit killer, but bear in mind that the magazine capacity is only 5 rounds. Plus, the zoom is too powerful for normal use. For those reasons, I much prefer the (less powerful) Dragunov myself. I'd advise saving the game before you buy it so you can give it a 'test run' rather than waste diamonds on a firearm you won't use regularly. It might come in handier later, I don't know, cause I'm still one third through Act 2.