We've had this discussion before -- if your hero/heroine doesn't know from hardware, no problem, say "Gun" and "went bang." and you are good. But if your sleuth or action hero is supposed to be able to field-strip a tank underwater in the dark while fighting off sharks amidst clouds of spewing oil, then it is a good idea to know a bit more so as not to sound ignorant when you write that scene.
So, one of the writers in the group took the class and then reported back on it. The writer was doing fine, talking about stance and grip and sight picture and all, until he got to the Dirty Harry part.
At the end of the session, he said, the teacher brought out a S&W .44 Magnum revolver -- Dirty Harry's carry piece -- and they got a chance to shoot it. Got to put the bullets into the magazine, aimed, and --
Hold up there a second, Sparky. Magazine? Revolver?
Yep. That's what the writer said.
Now, to confuse things, there was a .44 Automag pistol used in one of the later Dirty Harry movies. And it did have a magazine. But the writer said "revolver." Of course, maybe that's not what he meant; however, it is one or the other, but it surely not both in this case. (There have been "automatic revolvers." Not in .44 Magnum.)
Cowboy shooters call their hog legs "pistols" and that's now fairly acceptable. However: sidearms that ride in holsters or stuck in a pocket or belt aren't all technically pistols, since this term means the breech is contiguous with the barrel. The old dueling flint- or cap-locks were pistols, and when Sam Colt came along, what he made and sold weren't pistols, though people can be forgiven for making the generalization.
So, granted, this pistol-versus-revolver definition is hair-splitting and most people don't know and don't care. Ditto the difference between "clip" and "magazine." They aren't the same. And as more and more people continue to misuse it, it will eventually become the definition. In the grand cosmic scheme of things, it doesn't matter much. But even little errors will stop a reader who knows better and make them wonder: Gee, if he got something I know about wrong, what else is he getting wrong that I don't know about?
You want the reader to relax and trust you. If they don't, you are apt to lose them.
For me, catching a writer in a little mistake is usually not enough to kill my interest -- Lord knows I make enough of my own -- but now and then, it will. I'm not talking about a typo, but about something the writer assumed that s/he knew, but didn't. Schweitzer's Rule: If you don't know it, don't say it.
Take the word "hopefully." Most people get it wrong most of the time -- it doesn't mean "I hope." it means "With hope." -- and pointing this out gets you blank stares, because most people don't know the difference. But if your protagonist is supposed to be some kind of expert in a field, even if they dismiss something in passing as unimportant, they have to demonstrate expertise. You telling me they are an expert doesn't cut it. Show me.
In no definition of "magazine" pertaining to firearms does that word get applied to the cylinder of a revolver. That's just wrong, and if you put that in a book using a protagonist who is supposed to be dead-eyed death with anything that goes bang, that's when you lose my trust.