The .22 has been a favorite of hit men partly because it makes less noise and is even quieter with a silencer. The traditional mob coup de gras is 3 in the head, at least in Sicily.
The book, " The Hit Team ", by Dag Christenson describes a special .22 Beretta that the Mossad uses. They shoot the victim multiple times, so he is unlikely to survive. The Israelis are believed to have assassinated Gerald Bull, who was shot about 14 times with .22s. The book, " Vengeance " by Jacobs and a recent book about revenge for the Munich massacre deal with the subject of Israeli hit teams also.
A classic .22 failure was the attempted hit on a Libyan dissident living in the U.S. A former Green Beret shot him in the head with a .22 magnum but he survived. The story is detailed in the book, " Manhunt " by Peter Maas.
Years ago, a man was robbed and beaten in San Franciso and showed up at a hospital complaining of a headache. An X-ray revealed 5 .22 bullets lodged in his head. One of the bullets missed a vital area by a half-inch. The story was in all the papers.