One of my favorite authors is John Sandford (aka John Camp). He writes police procedurals and generally gets gun stuff correct (IIRC he had the police beat when he worked for the St. Paul newspaper and was in the Army (airborne (I think) and a stint in South Korea). He’s written over 50 novels and once in a while a makes a mistake. Here’s his explanation of how that happens. (It's an 'afterward' included at the end of his book Ocean Prey.)
I would highly recommend any of his books but would start with his 1989 book Rules of Prey.
My brother-in-law Dan called me up and told me that in Golden Prey, I’d referred to a 40mm pistol. Should have been .40 caliber. Calibers are hundredths of an inch, millimeters are …millimeters. A 40mm pistol would shoot a bullet about an inch and a half across. There is a 40mm round---it’s fired from a grenade launcher, not the kind of weapon that Lucas Davenport would have tucked under his sports coat.
How do these mistakes happen? It’s not usually ignorance. They arise out of all kinds of things…haste, changes in story, weariness, boredom, juggling too many nouns at once. In another Prey novel, I had a man click off the safety on his Glock 9mm pistol, stolen from a Minneapolis detective, before he entered a house. The 9mm was fine, except Glocks don’t have safeties.
I’d originally written that the man had been carrying a Beretta, which do have safeties. Then, I made the mistake of talking to a Minneapolis detective who told me there’d been a change of policy, and they were no longer allowed to have personal carry pistols. They were required to use issue pistols, which were all Glocks. So, trying to be accurate, I changed “Beretta” to “Glock”-this was after the novel was essentially finished-and forgot that several lines above that, he’d clicked off the safety.
In Winter Prey, on the first page, I have a snowmobiling villain following a compass course of 375 degrees through a blizzard. That’s tough since compasses only have 360 degrees. It was supposed to be 275 degrees, or west, but instead, he’s going northeast. I don’t know how that mistake occurred, but it should have been caught by somebody, at some point. I suspect it was a pure typo.
The thing is, I know about guns and have been shooting since I was in elementary school. I know the difference between millimeters and calibers. I know Glocks don’t have safeties. I know how many degrees there are on a compass.
Holding a hundred thousand words in your head, through numerous edits and rewrites, is a complicated business, and by the time you get to the end, you can barely stand to read through them again. When you’re dealing with numbers, especially they can jump up and bite you in the ass.
I would highly recommend any of his books but would start with his 1989 book Rules of Prey.