In San Diego County, which is the focus of this article, police agencies issue and use everything from the .38 revolver to the .45 semi-automatic. What weapon an agency decides to use is based on a number of factors, including costs associated with weapons and ammunition, weapon maintenance, ammunition needs and goals, ability of officers to handle the weapon and ammunition, and product support received after the purchase of a weapon. In terms of the effectiveness of a particular firearm, the author notes that there are two types of immediate "stops" from a bullet: one that disrupts the central nervous system by penetrating the brain or the spine and one due to massive tissue disruption. Shots that disrupt the central nervous system are caused more by shooting accuracy than caliber of weapon (size of bullet). Stops caused by massive tissue disruption are dependent on caliber, since deep penetration and disruption of vital organs is required. In tests observed by the author where the 9 mm and .40 caliber bullets used by police agencies were fired into gelatin, there was very little difference in effectiveness as measured by penetration and disruption. The author cautions about changing departmental weapons, since in the field officers will act instinctively according to the training they have received with a weapon. Officers who have been trained thoroughly and over a long period with one weapon will tend to react best with that weapon and fail to react appropriately in accordance with the characteristics of a new weapon issue.