Instructions
Travel to an area close to a Gang Hideout location.
Save your game to create a restore point in case you fail to pass the Sharpshooter challenge.
Equip the Mauser pistol and fire the gun until you only have 15 bullets left. This ensures that you're unable to reload the gun, as a single clip carries 15 bullets.
Enter the Gang Hideout and find a safe place to take cover.
Initiate Dead Eye Aim mode to slow down time. Place the cursor over an enemy's gun and press the "Fire" button to disarm him. Repeat this process five more times to complete the Rank 10 Sharpshooter challenge.