The FBI recently completed a major study of shootings of police officers. Titled "Violent Encounters: Felonious Assaults on America's Law Enforcement Officers," the document is not currently available on the web. The publication Force Science News, which comes from the Force Science Institute, of the University of Minnesota, Mankato, has reported on the study. Regarding firearms, FSN writes:

"Predominately handguns were used in the assaults on officers and all but one were obtained illegally, usually in street transactions or in thefts. In contrast to media myth, none of the firearms in the study was obtained from gun shows. What was available 'was the overriding factor in weapon choice,' the report says. Only 1 offender hand-picked a particular gun 'because he felt it would do the most damage to a human being.'
Researcher Davis, in a presentation and discussion for the International Assn. of Chiefs of Police, noted that none of the attackers interviewed was 'hindered by any law--federal, state or local--that has ever been established to prevent gun ownership. They just laughed at gun laws.'"

The summary of the study also provides information that many of the criminals who attack police officers are fairly skilled at gun use, and, unfortunately, diligent in their training.

The FBI website says that "Violent Encounters: Felonious Assaults on America's Law Enforcement Officers is available from the UCR Program Office, FBI Complex, 1000 Custer Hollow Road, Clarksburg, WV 26306-0150 or by calling 888-827-6427." I tried the 888 number once, and got a recording.