Randy Disher states that the shooter used a Weatherby Fibermark rifle. Then he and Monk agree that it is a British rifle used by snipers and mercenaries. The rifle is actually a hunting rifle made in Paso Robles, CA.
The goof item below may give away important plot points.
In drawing his conclusion about who hired the assassin Monk bases his conclusion on the fact that no witness could have accurately said where the shot came from because of the echo effect in the area. It is plausible however that a witness could guess the direction of the shot from the wound position on the body and the way in which the victim fell, annulling Monk's theory.