Vince Flynn, author of the excellent Mitch Rapp series, almost always gets the guns right. Douglas Preston and Lincoln Child are also pretty good; their main character, Pendergast, carries a Les Baer .45.
On the other hand are the writers such as the one who said a 20-gauge shotgun would "knock a charging bear backward."
Lots of times these things DO depend on the knowledge of a copy editor. I made my living as one for 33 years and, if a particular subject was not my area of expertise, I made darn sure I didn't change anything without thorough research.