If you rely on the FBI Uniform Crime Reports (UCR) for numbers of justifiable homicides, the numbers will always be significantly under reported. First, in many years, many states simply do not submit numbers, so they are not counted. Not all police jurisdictions in other states submit numbers, so the total is always going to be low.
The definition of justifiable homicides vary from state to state. What is reported is often politically determined.
Here is the FBI UCR definition for justifiable homicides, from pages 17 and 18 of the Uniform Crime Reporting Handbook:
Certain willful killings must be classified as justifiable or excusable. In UCR, Justifiable Homicide is defined as and limited to:
• The killing of a felon by a peace officer in the line of duty.
• The killing of a felon, during the commission of a felony, by a private citizen.
NOTE: To submit offense data to the UCR Program, law enforcement agencies must report the willful (nonnegligent) killing of one individual by another, not the criminal liability of the person or persons involved.
The following scenarios illustrate incidents known to law enforcement that reporting agencies would consider Justifiable Homicide:
15. A police officer answered a bank alarm and surprised the robber coming out of the bank. The robber saw the responding officer and fired at him. The officer returned fire, killing the robber. The officer was charged in a court of record as a matter of routine in such cases.
16. When a gunman entered a store and attempted to rob the proprietor, the storekeeper shot and killed the felon
NOTE: Justifiable homicide, by definition, occurs in conjunction with other offenses. Therefore, the crime being committed when the justifiable homicide took place must be reported as a separate offense. Reporting agencies should take care to ensure that they do not classify a killing as justifiable or excusable solely on the claims of self-defense or on the action of a coroner, prosecutor, grand jury, or court.
The following scenario illustrates an incident known to law enforcement that reporting agencies would not consider Justifiable Homicide:
17. While playing cards, two men got into an argument. The first man attacked the second with a broken bottle. The second man pulled a gun and killed his attacker. The police arrested the shooter; he claimed self-defense.By this definition, many justifiable homicides will never be reported to the FBI.
Update: Gary Keck finds that justifiable homicides are between 5.6% to 13% of all homicides reported to the FBI. Another study backs him up:
The rarest, but most serious form of self-defense with a gun is a defensive killing. The FBI does not publish statistics on self-defense killings per se, but it did start publishing counts of civilian justifiable homicides gathered through their Supplementary Homicides Reports program in their 1991 issue. For a variety of reasons, the FBI counts of civilian justifiable homicides represent only a minority of all civilian legal defensive homicides. FBI-counted civilian justifiable homicides were used to estimate total civilian legal defensive homicides. FBI counts of police justifiable homicides are also reported here. Regardless of which counts of homicides by police are used, the results indicate that civilians legally kill far more felons than police officers do. The figures imply that, of 24,614 civilian (not by police) homicide deaths in the United States in 1990, about 1400 to 3200, or 5.6% to 13.0% were legal civilian defensive homicides.
This estimate was independently confirmed by the only national study of homicide dispositions done to date. Analysis of 231 homicides occurring in the U.S. in the first week of May, 1989 indicated that between 15 (6.5%) and 28 (12.1%) were ruled justifiable.