Many years ago, when I lived in Iowa, a post-office carrier knocked on my door and handed me a special-delivery letter, for which I had to sign a receipt. The letter came from South Africa. I didn't know anyone there, so with great curiosity I opened the envelope and discovered a message from a combat school. An angry message. The teachers and students had been fans of my novels for a long time, I was informed, but they were fans no longer because my latest book made clear that the creator of Rambo knew nothing whatsoever about firearms.

Two hours later, a different post-office carrier knocked on my door and delivered a second special-delivery letter. Again I signed a receipt, and again I noted that the letter came from South Africa. It was from a combat school as well, but from a different combat school. The teachers and students there had been fans of my work also, but they, too, vowed never again to read a word of my wretched prose because it was obvious that I had never fired a handgun in my life and knew nothing about them.

These letters surprised me, not only because they came from so far away and had the same angry message but also because I had, in fact, a lot of experience with firearms, particularly handguns, and did my best to double-check the facts about anything I wrote, especially firearms.

What on earth were these knowledgeable schools complaining about? To my shock, when I looked at the passages they cited, I realized that the schools were right. But it wasn't my fault. Well, not exactly.

This is what had happened: In my then-recent novel (which I won't name because I don't want anyone looking at the book for the wrong reason, and anyway in most subsequent editions the problem was fixed), I had foolishly decided at the last minute (the very last minute, because the novel was at the galley stage, my final opportunity for corrections) that there were a lot of semi-automatic handguns in the book and for variety maybe I should change one of them to a revolver.

(As an aside, let this be a cautionary tale about not making changes at the galley stage, when a copyeditor will no longer be available to make certain that the changes are consistent with the rest of the text.)

So, presto, a semi-automatic pistol (the ammunition is in a magazine inserted into the weapon) became a revolver (the ammunition is in a rotating cylinder).

 But I didn't think to change any other details. Thus, in a major action scene, as my protagonist prepared to scale a wall, he pressed the revolver's safety catch. Later, he released the revolver's safety catch.

Writing those words, I grit my teeth. My chest tenses. My face turns warm with shame. Aarrgh.

Revolvers don't have safety catches (at least none that I'm aware of, but I've learned that if I make absolute statements about fi rearms, someone will find an exception). There is indeed a button at the side of some revolvers, but its function is to allow the ammunition cylinder to swing open for reloading. When my protagonist pressed that button, in all likelihood he would have caused the cylinder to open, dumping the ammunition onto the ground.

The moral is—when you write about firearms, you can't be too careful.

You can kill a dog in one of your novels, and you'll receive angry e-mails (no need for someone to send a special-delivery letter from a foreign country; these days, an e-mail creates instant gratification). You can make a mistake about a motor vehicle, and you'll definitely hear about it. You can make a mistake about a particular type of horse or the distance between two cities or a sports statistic, and you'll hear about it, and you'll probably receive a one-star review on Amazon because of it.

But no wrath is greater than that of fi rearms enthusiasts. Even if you've had instruction, Benjamin Sobieck's The Writer's Guide to Weapons: A Practical Reference for Using Firearms and Knives in Fiction is going to save you a lot of time apologizing and vowing to do better in response to a barrage of e-mails from irate readers.

Of course, the best way to learn about fi rearms is to contact a local sporting-goods store and sign up for a course. I'm baffled why some authors who write about fi rearms don't do this. At the least, they'd discover that a gunshot is loud and gives off a distinctive acrid odor—sensory details often omitted from gunfight scenes written by authors whose only experience with fi rearms is in watching movies.

How many times have you watched an action fi lm in which a villain threatens someone by pulling back the pump-action on a certain type of shotgun and then doing it again, and again, never firing? The repetition apparently is meant to emphasize that he means business.

The only problem is that the pump-action on a certain type of shotgun inserts a shell into the firing chamber. The next time the pump is used, the shell in the fi ring chamber is ejected while a new one is inserted. Thus, in movies that show a character "racking" a shotgun repeatedly without ever firing it, shells ought to be flying onto the floor. Ditto for Western movie characters who repeatedly work the lever on a rifle without firing it, evidently intending to indicate their intense emotions. Again, in the real world, perfectly good ammunition would be flying comically through the air. Ditto for a character who repeatedly racks the slide on a semi-automatic pistol without fi ring it. Perfectly good ammunition would be accumulating at the character's feet.

I was pleased and amused to see Benjamin Sobieck emphasize these and many other mistakes. "Don't" is oft en as good an instructional technique as "do." For authors, regardless of their sophistication about these matters, the clarity with which this book explains many kinds of firearms and knives is a gift . The differences between semi-automatic pistols and revolvers, between the types of calibers, between magazines and clips (I once heard someone say he would never read a famous thriller author again because that author referred to a magazine as a clip)—these and similar danger zones for writers who use firearms in their novels can now be entered without fear.

David Morrell