There is an infamous book called 'Hitman' published in the 1980's I believe, published by Paladin Press, that claims to be a manual on how to be, surprise surprise, a hitman. It recommends using a .22 for its ease of suppressability. It's a rather stupid for the rest, but what made it famous is that someone committed a murder in the way described in the book and after that the published was sued and the book got taken off the market. See
http://www.francesfarmersrevenge.com/stuff/archive/oldnews5/... .
I don't know anything about hit squads using .22, but in the (rural) area I'm from, .22 rifles are known as poachers rifles because when used with subsonic ammo and a homemade suppressor, they are very quiet.
That said, I'm not sure that hit squads are very concerned with being quiet. That Iranian virus expert that got killed the other week was blown up, the Israelis that killed the Hamas leader a while ago didn't even wear disguises and were filmed full frontal. From the targeted killings that make the news, being 'subtle' doesn't seem to be a consideration.
Oh and drug dealers who kill people don't care about being silent, either. All high profile shootings of the last few years in my area were with heavy caliber weapons.