At the start of the mission, ignore the man walking away from you. Wait by the front desk until a customer comes over and starts talking to the man behind the counter. When he does, crouch down and go behind the counter. Grab the keycard from behind the counter and then go through the doors on your right. Go through the back room and come out the other side.
You'll be outside in the trench that runs along the outside of the firing range. Follow the trench to the left but keep your head down or you might be seen/heard. Continue around the trench until you find a door carved into the rock on your right. Use the keycard to get in through the door. Inside, you'll find somei interesting weapons, a notebook with a safe combo, and a piece of Evidence. Pick up and put away the weapons, grab the notebook and the evidence. Leave the trench.
Continue along the way and grab the Kazo Sniper Rifle on the left. Now head back to the front of the trench so that you are under the shooters. Take cover and slide left. When you get to the far left, transfer cover. Slide along the building and make sure that there's nobody inside the room through the window. When it is safe, hop in the window, open the safe and grab the key. With the key in hand, go through the vent in this room to get back to the gun shop.
Inside the gun shop, pick up and put down as many weapons as you can before making your way back to your Silver Ballers. Simply open the case and claim them. Leave through the front doors and you're done.
