Suppressors

Paul A. Clark, "Criminal Use of Firearm Silencers," Western Criminology Review 8(2), 44–57 (2007):

The Lexis/Westlaw database contains 153 cases over the past ten years in which the evidence suggests a silencer was used for a criminal purpose — including unlawful possession of a silencer where no other crime was committed. That gives an average of about 15 reported cases each year, and assuming this represents close to half of all prosecutions, one can assume 30-40 total cases per year. This is out of 75-80,000 federal criminal prosecutions each year. Overall numbers certainly suggest that silencers are a very minor law enforcement problem.

Moving from the overall numbers and looking at more specific offences, it appears that use of silencers in truly violent crime is even more rare.