DISCLAIMER:
I am not an expert on this subject, and if a real expert wants to submit an article I will gladly defer to it. I have been interested in this subject for about 10 years and I have paid attention to magazine articles, Internet chat, books by the Ascended Masters (such as Ayoob, Farnam, and Cooper), and so on, as well as accumulating plenty of personal experience with both semi-automatics and revolvers. In this article I have tried to present what appears to me to be the conventional wisdom, with my own personal opinions clearly identified as such.
DISTINCTION:
When I talk about reliability in this article, I am talking about the possibility that a handgun becomes suddenly inoperable (a major embarrassment during a self-defense emergency). I am not talking about the expected shooting life of a handgun. All handguns are mechanical mechanisms which will wear out if fired enough times, and all handguns should be examined periodically to assure that all parts are within manufacturer's specifications. Consult your owner's manual for suggested maintenance procedures. Durability of handguns (expected number of shots you can fire before needing to replace parts) varies all over the map, from the zinc-alloy guns which are lucky to survive 500 rounds, up to the Glock pistols which seem to be good for at least 500,000 rounds of +P+ ammo.
BOTTOM LINE:
The bottom line is that revolvers, as a class, are more reliable (less likely to become suddenly inoperative) than are semi-automatic pistols as a class. Nonetheless, with proper attention to detail, certain semi-automatic pistols can be made sufficiently reliable that they become more system-effective as self-defense tools than any revolver. The balance of this article will examine those details.
REVOLVER RELIABILITY:
First, I will briefly dispose of the reliabliltiy problems typical of revolvers. Most revolvers are more fragile than the best semi-automatics. For instance, if you drop a revolver onto a hard surface (so that it lands on its side), there is a non-negligable chance that the cylinder will become jammed, and will only be unjammable by a gunsmith with a supply of spare parts. There are also two classical revolver jamming scenarios connected with reloading. In the first, you eject empty cases, but one of them doesn't come all the way out of the chamber. Not noticing this, you release the ejection rod, and the ejector star pushes the empty case back into the chamber. Now the cylinder will not close because the ejector star will not lie down flush, and you must extract the offending case with a rod or other tool, by which time the gunfight is over one way or another. The second is closely related: during reloading, you get chunk of unburned powder, or some other foreign object under the extractor star. The cylinder may or may not close, and if it closes, it may be too tight to turn. You can fix this problem by emptying the cylinder, pushing out the ejector star, and carefully wiping the star and cylinder face with a clean cloth, but again, this takes too much time to be practical. If the cylinder will close but not turn, you can try to manually cock the hammer (using both hands, or by pushing the hammer spur against some object), and if you can get the hammer cocked, firing a round will hopefully squash the offending object flat enough to allow the gun to operate normally. Anyway, assuming you don't drop your revolver and you have loaded it with fresh factory ammo which has been protected from oils and solvents, you can count on firing all of the cartridges in the cylinder at the very least. This is a high degree of practical reliability.
SEMI_AUTOMATIC RELIABILITY:
Semi-automatic pistols have two major sources of unreliability which revolvers do not share - the automatic feeding of fresh cartridges into the chamber, and the automatic ejection of empty cases. Failures to feed are related to the exact geometry and surface characteristics of the parts of the pistol which the fresh round touches during its journey into the chamber, the geometry and surface characteristics of the fresh round itself, and the force with which the slide moves back and forth (this last factor is influenced by the shooter's method of gripping the gun). Failures to eject are influenced by the design of the extractor (the claw-shaped part which pulls the cartridge case out of the chamber), the surface characteristics of the chamber and cartridge case, and the geometry of the ejection port.
Failures to feed and eject can also be caused by any mechanical interference with the normal operation of the pistol's moving parts. For instance, care must be taken when shooting a semi-automatic pistol from behind cover. If the slide of the pistol touches a wall or other object, a jam of some sort is likely. If the ejection port is blocked in any way (by an adversary's hand grasping the gun, by clothing, by proximity to a wall or other cover, etc) a failure to eject is almost guaranteed. Firing the pistol with the muzzle in contact with an adversary is likely to cause a jam of some sort.
Anyone who owns a semi-automatic pistol should learn and practice drills for correcting the more common failure modes for his pistol. Opinions differ on what these drills consist of exactly, but for the record I will set down what I personally do (as taught by Greg Hamilton, an excellent instructor in Seattle):
If the slide is locked back, look or feel down the ejection port to see if you have run out of ammunition. If so, take out the old magazine, put in a new one, and keep firing.
Otherwise, the first response is "tap-rack-bang" - turn the pistol 90 degrees, so that the barrel is still pointed at the adversary but the ejection port is pointed down, then hit the bottom of the magazine firmly with the non-shooting hand, then grasp the slide (so that the ejection port is not blocked), pull it fully to the rear, and release it. Then re-aim the pistol and attempt to fire it. If the pistol still doesn't fire, the second response is "lock-rip-work-tap-rack-bang" - lock the slide back, remove the magazine and hold it with your shooting hand's little finger, turn the pistol so that the ejection port is down, grasp the slide (so that the ejection port is not blocked) and vigorously move the slide back and forth three times. Then reinsert the magazine, pull the slide all the way back and let it go, re-aim the pistol, and attempt to fire it again. If that doesn't work, actually look at the pistol, diagnose the problem, and correct it. Diagnosing the problem takes longer than you would expect, especially under stress, so it should be your last resort. The two responses above will solve most problems with most pistols.
Please note that these techniques are for use in self-defense emergencies only - if your pistol fails to fire unexpectedly, at the range or in some other non-threatening situation, you should wait 30 seconds and then open the breach carefully, in case you have a hang-fire (a cartridge which is taking it's ownsweet time about firing, but will get around to it eventually). I strongly advise you to attend a good shooting course which covers these techniques.
MAKING YOUR SEMI-AUTOMATIC RELIABLE:
The first and most important consideration is to obtain a pistol which is capable of high reliability. Since the reliability of semi-automatic pistols depends upon small details of design, it is not surprising that different makes and models have very different levels or reliability. I think it is safe to say that there are about 10 companies which have developed a reputation for making reliable pistols. Those are, IMHO: Ruger, Colt, Smith&Wesson, Browning, Beretta, Walther, Heckler&Koch, SIG, Glock, and Taurus (not in order). Beyond that we get into religious issues, but for the record, in my own experience Glocks, SIGs, Berettas, and the 3rd generation S&Ws are the most reliable. You will notice that these are also among the most expensive pistols on the market - reliability is the result of careful design and meticulous manufacturing, and these do not come cheap. Every once and a while you will read about someone whose $89.95 wonder-gun has fired 50,000 rounds without jamming once. Even if this is true, it does not mean that you could buy the same model and expect the same reliability. Cheap pistols are manufactured to forgiving tolerances, and the next one off the line probably jams on every third round.
Be sure that your pistol is "ramped" and "throated". This means that the feed ramp (the slanted part leading up to the chamber, which guides the cartridge noses along) should be polished like a mirror, and the rear edges of the chamber itself should be rounded or beveled. Nowadays this work is usually done at the factory, but on some older or cheaper pistols you will need to have it done by a gunsmith.
Keep your pistol clean. Dirty guns jam, because dirt increases mechanical friction and thus decreases the energy available to execute the extraction and feeding operations. Your pistol should be field stripped, cleaned and lightly oiled (according to manufacturer's instructions) after every shooting session, or every few months if it is just lying around not being shot. In addition, your pistol should be detail stripped (totally disassembled) and cleaned every 1000 rounds or so. Many people pay a gunsmith to do this, but I do it myself using "The Gun Digest Book of Firearms Assembly/Disassembly, Part 1, Automatic Pistols", by J.B. Wood - $17 at many gun stores and some book stores, and a fine bargain.
Use factory magazines. The magazine is a major part of the feeding system, and small changes in magazine geometry can lead to big changes in reliability. It is possible to save money with aftermarket magazines, but they are often not as durable, and seldom as reliable. Practice with the same magazines you load for self-protection. Keep magazines clean (inside and out) but keep the insides free of oil.
Select your ammo carefully. You should always use fresh (i.e. less than 5 years old) factory ammo from a reputable manufacturer, when loading a pistol for self protection. Use whatever cheap junk you like for practice - the cheaper and more off-brand the ammo, the more jam clearing practice you will get. IMHO the reputable manufacturers are Federal, Winchester, Remington, CCI/Speer, Cor-Bon, and Hornady. In addition, you will need to select a particular type of self-defense ammo, test it for reliability, and then load only that type for self-defense. Most authorities recommend 200 consecutive shots without a single failure of any kind before a particular type of ammo is accepted for self-defense use. Note that when I say "type" of ammo, I mean a specific load from a specific manufacturer - e.g. for my 9mm Glock I use Federal Nyclad 124-gr HP. The Federal Hydra-Shok 124-gr JHP has identical ballistics and very similar shape, but nonetheless it would need to be tested separately for the full 200 rounds. This testing requirement makes most exotic ammunition (Glaser, MagSafe, etc) too expensive to consider, but that is no loss because exotic ammunition seldom has the same level of quality control one associates with the reputable manufacturers listed above, and it generally doesn't work any better than traditional hollowpoints.
SIGs and Glocks seem to eat just about anything, but most other pistols display definite likes and dislikes for ammunition. These derive partially from design and partially from manufacturing variations in individual guns, so it is not enough to ask what everyone else shoots in your model - you must still conduct tests for your individual gun. You may need to test several types before you find one that your pistol likes. There are many arguments about what types of ammo have the most effect on an adversary, and these are picked over endlessly on rec.guns and in other places in the FAQ.
[FAQ Maintainer: V.G.1. Ammunition for the Self-Defense Firearm is a good article on the subject]
For now all I will say is that reliability is more important than effectiveness. Start your testing program with the ammo which you believe will be most effective, but if it is not 100% reliable, try something else. As a very rough guide, the closer a bullet's shape resembles the standard FMJ load in that caliber, the better it will feed, and the closer a cartridge's ballistics match the standard load in the caliber, the better it will cycle. A tradeoff ensues because aggressively cone-shaped hollowpoints are generally thought to be more effective, whereas FMJ usually has a gently rounded shape, and because there is currently a fad for +P (extra high pressure) defensive ammo. To repeat, reliability is more important than effectiveness.
Rotate your ammo regularly. The round in the chamber and the top round in the magazine will get chewed up by normal adminstrative handling (clearing and reloading, press-checking the chamber), and should be demoted to practice status when they start looking noticeably mangy. Rounds that have been stored in a car should be demoted after a few months, because cars get hot sitting in parking lots (and heat speeds up chemical degradation of ammo) and because vibration can cause the powder grains to rub against each other and grind themselves down, which in extreme cases has even resulted in guns blowing up when the ammo was fired. Demote immediately any cartridge which has been exposed to oils or cleaning solvents - these can soak into the cartridge and damage the powder or primer. Demoting self-defense ammo to practice status is also a good thing because it allows you to run a continuous testing program to make sure that the manufacturer has not changed the recipe on you in ways that effect reliability.
Use a firm grip. Semi-automatic pistols depend upon the mass of the frame (and whatever is holding the frame, i.e. your hand) to resist rearward movement during recoil, so that the rearward movement of the slide instead compresses the recoil spring to power the next feeding cycle. If you hold the pistol too loosely ("limp-wrist" it), which is tempting because it makes recoil hurt less, you will reduce the mass available to oppose recoil, and thus you will reduce the amount of energy stored in the recoil spring. In pistols with very light frames (especially the Glocks) this effect is much more pronounced. If the gun barrel ends up pointing more than about 20 degrees above the horizontal due to recoil, you are getting into limp-wrist territory. If your pistol is jamming even with FMJ ammo, you are very likely having this problem - try a rested two-handed hold, or ask someone with strong wrists to try shooting it. If you are apparently having limp-wrist failures, your three choices are either to grip the pistol more tightly, or to get a different semi-auto (probably a heavier one which shoots less powerful ammunition), or to get a revolver.
If you do all of the things above, you can get a semi-automatic pistol to be sufficiently reliable that other factors dominate your chance of surviving in self-defense situations. It is generally accepted that semi-automatic pistols are easier to shoot fast and straight when under stress, and this factor alone should dominate if your pistol doesn't jam more than a few times per hundred shots. If your pistols only jams a few times per thousand shots, which is achievable with reasonable effort and expense, then you have as much reliability as you need.