It is not a so difficult achievement as this.
Towards the back of the carnival is a shooting range. You have the option to shoot Thieves, a Sheriff's badge, a woman, and a green bottle of whiskey. All other options move side to side, the bottles are the only ones that move up and down.
If you focus only on bottles, you can easily attain an average of 20 bottles by game.
-----> Credits : SengIV from www.xbox360achievements.org <-----