Revolvers are more reliable than semi-autos. There's a poem floating about: "Six For Sure." The oft repeated wisdom is that revolvers have simpler construction and few parts which means that there are fewer things to break.
Except that's a myth.
Revolvers have more parts than semi-autos more often than not. Revolvers require higher precision and tighter engineering in the lockwork or they will be "out of timing" which can cause problems including shaving bullets (and spitting shards out the cylinder gap), jamming up the mechanism and not turning, even catastrophic failure (aka "Kaboom"). Revolvers are precision instruments which must be properly tuned or they will fail. If parts of a revolver need replaced, such as the cylinder or hand, an experienced gunsmith is usually required to custom "fit" the parts to that specific machine. In essence, revolvers are a finely engineered clock that can either spit out bullets when properly constructed or explode when ineptly designed and constructed. But revolvers do have a significant advantage in that they are very forgiving of large variances, out of tolerance construction, and even many significant failures, of ammunition. Different bullet weights, pressure levels, and even failures to ignite (a "dud" round or "misfire), are all taken in stride with a revolver. In contrast, semi-auto pistols are often much more impacted by variances of pressure and a misfire is not so easily resolved. In other words, a well constructed revolver is ideally suited to use ammunition which may have less quality control than may be offered by 21st Century industrialized mass production methods.
Semi-auto pistols very often have fewer parts than revolvers. Additionally, those parts very often can mate less perfectly and still allow the pistol to function reliably. This is particularly true of the mating between the slide and the frame. Often large and visible gaps between slide and frame or rails, sometimes called "loose tolerances" or, less charitably, "slop," will not impede function of the semi-auto. Loose tolerances can sometimes actually improve reliability by making the machine less vulnerable to environmental contamination such as by dirt or powder residue, or even simple usage wear. Replacing parts on most semi-auto pistols today is a simple matter of "drop-in" replacements which many owners can conduct on their own without requiring a trained gunsmith. However, semi-autos are far less tolerant of variances in ammunition. Different shaped bullet profiles, such as flat point, round point, or hollow point, variances in over-all length, or variances in pressure levels, can cause a semi-auto to fail to function properly or be damaging to the machine. Additionally, a misfire requires special steps to load a new round into the chamber which is not required of a revolver. In other words, while a semi-auto regularly has fewer parts to break and often requires less precision in design and construction, it also requires more precision and consistency from ammunition.
With the rise of industrialism, particularity the ever increasing precision and consistency of production methods starting roughly around the mid-20th Century, the higher quality ammunition allowed for less precision of the pistol.
In other words better production methods of ammunition allowed for an inversion of the design philosophy of the machine. Improvements in the quality of ammunition allowed firearm designers to step away from engineering the proverbial "Swiss watch" to simpler machines. The cost of ammunition, on a "per round" basis, remained low and became lower because of manufacturing improvements, which allowed them to design and build less complex, and therefore less expensive, more easily produced, and more easily maintained, machines.
[Glock G17 semi-auto pistol - 34 Parts]
[Kel Tec PF9 semi-auto pistol - 40 Parts]
[Ruger P345 semi-auto pistol - 47 Parts]
[Colt "Government" 1911 semi-auto pistol - 52 Parts]
[Ruger SP101 revolver - 65 Parts]
[Ruger GP100 revolver - 70 Parts]