Of all the guns you may own, the one you carry for self-defense is the one to which you should pay the most attention. Unfortunately, the opposite is usually more accurate. The "always" gun (as in always with you) gets the least attention because you have probably already established its reliability — or you certainly should have — and are now taking it for granted.
The problem is, there's a bit more to do on an ongoing basis past checking to see if the gun is loaded or empty and giving it a quick brush and puff to remove any clinging debris or clothing lint. The cartridges need regular examination, the bore (and the cylinder if it's a revolver) examined for any foreign objects.
If the gun is not carried but permanently positioned, as might well be done with a home-defense gun, you should also check for any "critters" that found the bore a good place to raise a family.
For semiautos, determining the condition of the cartridges you load is critical. How many of us regularly chamber and re-chamber the first two rounds of our carry loads? Each time you chamber or clear a round it is bumped as it loads or is cleared. As this happens, so does "bullet pushback" and "crimp-jump." Also, the case rim and extractor groove get altered slightly.
Pushback happens when the bullet is loosened in its case by repeatedly striking the barrel feed ramp and interior of the cartridge chamber. This allows the bullet to then be both pushed back or moved forward (crimp jump) against the tensioning done to the cartridge case neck where it envelops the lower circumference of the bullet to hold it in place.
As to the effect of even relatively slight backward movement, chamber pressure increases due to the powder now burning in a smaller space, and this increased pressure can reach a catastrophic level.
How much is "slight"? In 2004 I read a report from Hirtenberg Ammunition Company (produced at the request of Glock Gmbh) regarding the .40 S&W cartridge. The ammo company found that if the bullet was pushed back 0.1 inch, chamber pressure doubled.
More recently, this phenomena was further confirmed by Guy Neil, a ballistics expert who, in his column in the March/April edition of Front Sight magazin, noted that years ago Speer cartridge company found the chamber pressure of a 9mm round increased by 55 percent when its bullet was seated 0.033 inch deeper.
These pressures are significant. For comparison sake, an aerosol can of air used to clean a computer runs about 70 psi. Normal chamber pressure for a 9mm +P cartridge is 38,500 psi, and a proof load measures 55,500 psi.
The bullet can also move forward (the crimp jump mentioned earlier) due to inertia acting on it as the cartridge is slammed forward in the loading or firing cycle. I don't know of any pressure problems with this, but clearing such a round is difficult since, due to the added length (though slight), it no longer easily clears the ejection port window. Also, if loaded in the magazine, the longer round can wedge itself such that no rounds can be moved upward to be chambered.
The worst situation here is when the bullet separates from its case. I've experienced this using reloads in rifles and handguns when the bullet was not properly crimped.
With a revolver, normal loading will not cause bullet pushback, but cartridges can jump the crimp, lengthening to the point they protrude through their charge holes. Then, as the cylinder turns to bring up a live round, the protruding bullet stubs against the frame of the revolver, preventing it from turning.
Smith & Wesson addresses this in the revolver owner's manual, with particular attention paid to S&W ultralight revolvers. It is suggested that a simple firing exercise be done using one's selected carry ammunition. Fully load the cylinder, then fire all but the last round. Inspect this one to see if the bullet has started to move forward from its case (a caliper is handy for this; measure the cartridges as they come out of the box, then measure the last round to see if there's a difference). If this happens, try another brand or bullet weight and repeat until you find one where this does not happen.
Also, despite the higher prices now charged for quality ammunition, it is worth its cost as the ammo makers are well aware of the foregoing problems and make every effort to provide cartridges that can tolerate a few repeated cycles in a pistol. But don't be cheap. If there is any doubt, there is no doubt. Don't carry or fire questionable ammo and never shoot any range discards. (As Patrick Sweeney noted recently in these pages, most cheap, off-brand ammo is just that — cheap at best and just plain junk at worst.)
Bullet pushback or any bullet movement, along with case damage, cannot be ignored when shooting any firearm. Even the best ammo can be altered or damaged enough that when it is fired it could cause severe, if not fatal, injuries to yourself or others. Prevention is always the best policy.