SPOILER? honestly, don't you just hate all that sneaking, especially when you have NO saves, so if you blow it you have to start the whole, tedious process over from the beginning. and if you are like me, you stand a less than even chance of sneaking up on that bald-headed so and so (snappy dresser though). here's a quick way to SA:
forget going to the locker, you don't need that sniper rifle, Mr 17 will be providing you with a loaded one shortly. Run or walk to a vantage point south of the Pushkin building. If you're on the Normal or Expert levels you could save here. Don't peak around the corner though: 17 is in a 2nd story window near the front door and he'll drill you. Instead dash down the street to the right. Don't run straight to the building (he'll drill you); you're trying to provide a hard target to hit if he decides to take a shot. Sometimes he shoots, sometimes he doesn't.
Once out of the sight-line from the front window run to the building, in the front door, and take the steps up to the left to the room with Sergai's cardboard stand-in. Keep running! When you entered the building Mr 17 dropped his rifle and took off in pursuit: you're now chasing each other in a circle. Run to the room where he dropped the rifle, pick it up and continue in that direction into the room where you normally have to sneak up on him, run out the door, down the steps and out of the building to the vantage point south of the Pushkin building.
Pause to catch your breath and let 17 get back to the room where he dropped his rifle. he'll cooperate by standing still enough for a headshot (don't blow it: you only get one bullet). drop or carry the rifle to the nearest sewer entrance: the shortest route to the subway will avoid any guards. once you're in the subway station there's a chance one of the guards will be near the top of the steps leaving the locker room (which you can't determine without the guards on the map function); pause then run, or just run.