In the Harry Potter series, a squib is a child born to a magical family who somehow grows up without having any trace of magical ability herself. This is a child who didn’t become what her parents expected her to become.
In the gun world, a squib is a bullet that fails to exit the barrel when the round is fired. It gets stuck in the barrel and never becomes the projectile that you expected it to become when you pressed the trigger.
When a squib happens, you will usually hear a very strange sound. It’s not the BANG! sound you expect from a typical shot. It does not make the pew, pew, pew sound a politician might expect, either. Rather, it makes a kind of phthpht noise, or a quiet pop sound – somewhat muffled and weird-sounding. You will also feel an unexpectedly gentle recoil, or no recoil at all.
If you hear that sound on the range, you should stop shooting immediately to find out what’s wrong. Examine your gun to be sure there’s nothing stuck in the barrel. That’s important, because a bullet that’s stuck in the barrel can cause serious problems if you try to shoot the gun without clearing the stuck bullet out of the way first. Sometimes pressing the trigger again can cause serious, permanent damage to your firearm (think “bulged barrel”), and it can even cause an injury to you or others if the barrel breaks completely open from the pressure of the next shot.
However (and this is important!), if you ever hear that weird-sounding phthpht or pop sound in real life, when you are defending yourself from a violent criminal, you should keep shooting. Why? Two reasons.
First, because the danger of getting injured from the squib stuck in the barrel is very small compared to the danger that made you start shooting in the first place. In those circumstances, you don’t care about the damage to your gun; you’re just trying to save your own life. You may be able to “shoot the squib out of the way.” This is most emphatically not recommended for a calm day on the range, but when your life is on the line it does not matter if you damage your gun when you try it.
Second and more important, you keep shooting because auditory exclusion is one consistent feature that survivors of criminal encounters recall. What’s auditory exclusion? That’s when things don’t sound the way we expect them to sound. Some sounds are muffled, while others are exaggerated. One law enforcement officer recalls having another person fire a full-power, 12-gauge shotgun about three feet from his right ear during a violent event. But the officer never realized that his friend had fired the shotgun. He never heard it. The stress of the situation had affected his hearing.
Another person tells the story of hearing a weird, muffled pop coming out of her gun as she defended herself from a rapist. There wasn’t anything wrong with her gun, which we know because every round she fired struck the rapist. The shots just sounded weird to her because the physiological and psychological effects of defending her life in a high-stress situation had affected her mind’s ability to process the sounds she heard. She heard a pop where she expected a BANG! – but there was nothing wrong with her gun. She needed to ignore the weird sound and keep shooting to save her own life.
[Edited to insert a link to a relevant picture at Tamara's View from the Porch blog.]