It is recommended that you spray all 100 tags, as it is easier for ammunition and early on missions. Get a cop car (preferably a car, OT truck, rancher, etc.). Save it in the Johnson family garage. Save the game if desired. Get a large amount of sawn-off shotgun ammunition. If all 100 tags were sprayed, it is available in Johnson kitchen. Go to the garage. Get as far away from the car as possible without letting the garage door close. Then crouch, aim for the grill of the police car, and shoot. Every few shots (or every time the car catches fire), you should have more weapon skill. It will eventually upgrade about every minute. When it catches on fire, let the garage repair the car and come back. This does not work as well at getting Hitman on the other weapons.