Murder in the Gunroom. by H. Beam Piper

Jeff Rand, a private detective, is skeptical when he is employed by Gladys Fleming to evaluate her recently acquired gun collection, which happens to contain a dark secret. The more facts he uncovers, the more interesting the story becomes. Gun dealers, butlers, wives and cops all become suspects in the investigation of a mysterious death. The book is rich with detailed descriptions of the many different guns that star in this tale.
This is the only murder-mystery written by Piper, who was mostly known for his science fiction novels.