That was probably not exactly what he was expecting, to be quite frank. The bullet should have bounced off of his mask, most bullets do. High powered rounds being the exception, but he's generally too close for that to matter in most cases. It was not in fact the case here, where the bullet shot clean through his mask and punched a hole in his face. 

Not quite the hole he was expecting, as he had enough time to think about the mistake that he'd made and was pretty much prepared to die for that bullet. When it struck his face he was knocked backwards, either from the punch of the bullet or from reflex. Shin's weight slammed into the wall behind him, and his upper body was pushed through a window. His right arm passed through and his left caught on the edge. Glass cut dangerously through his clothes along his sides and back, blood seeped from them almost immediately. 

Even with his high pain tolerance he was half tempted to scream, but he had an idea. Taking the bullet to the face might be a good thing, despite all arguments to the contrary. For now, Shin held himself almost perfectly still, not even breathing. Fortunately for the assassin, his face was inside the building, and the tears of agony that were streaming up his face could drip out of sight.