In firearms, the cylinder is the cylindrical, rotating part of a revolver containing multiple chambers, each of which is capable of holding a single cartridge. The cylinder rotates (revolves) around a central axis in the revolver's action to sequentially align each individual chamber with the barrel bore for repeated firing. Each time the gun is cocked, the cylinder indexes by one chamber (for five-chambers, by 72°, for six-chambers, by 60°, for seven-chambers, by 51.43°, for eight-chambers, by 45°, for nine-chambers, by 40°, and for ten-chambers, by 36°). Serving the same function as a rotary magazine, the cylinder stores ammunitions within the revolver and allows it to fire multiple times, before needing to be reloaded.

Typically revolver cylinders are designed to generally hold six cartridges (hence revolvers sometimes are referred to as six-guns or six-shooters), but some small-frame concealable revolvers such as the Smith & Wesson Model 638 have a 5-shot cylinder, due to the smaller overall size and limited available space. The Nagant M1895 revolver has a 7-shot cylinder, the Webley-Fosbery Automatic Revolver has an 8-shot cylinder in .38 ACP, the LeMat Revolver has a 9-shot cylinder, and the Smith & Wesson Model 617 has a 10-shot cylinder in .22 Long Rifle.

As a rule, cylinders are not designed to be detached from the firearm (except for cleaning and maintenance). Rapid reloading is instead facilitated by the use of a speedloader or moon clip, although these work only on top-break and swing-out cylinder revolvers. Revolvers that have fixed cylinders must be loaded and unloaded one chamber at a time.

Fluted cylinders are commonly found on revolvers and go back to early revolving firearms, including the Elisha Collier flintlock revolvers. By fluting the cylinder, firearms manufacturers can reduce the weight of the firearm without compromising adequate strength. In addition to reducing weight, a fluted cylinder also allows a revolver's cylinder to be rotated by hand easier, which was important on early manually revolving firearms, as it was a helpful aid in loading and unloading. While pepperbox revolvers often had ribbed or fluted cylinders, during the percussion cap and ball era, most traditional revolvers had non-fluted cylinder. Samuel Colt and the Colt's Patent Fire Arms Manufacturing Company experimented with fluted cylinders on several models in the 1850s and 1860s, including releasing the Colt Model 1862 Pocket Police with a fluted cylinder as standard during the American Civil War. Smith & Wesson also released their Model No. 1 Third Issue and Model No. 1-1/2 Second Issue revolvers in 1868 with fluted cylinders followed by the Smith & Wesson Model 3 in 1870; thereafter, fluted cylinders were essentially standard on revolvers, including the famous Colt Single Action Army released in 1873 and famously used in the Indian Wars and Wild West. In more recent years, companies have produced revolvers with non-fluted cylinders for the purposes of more extensive engraving or for more robust aesthetics (often on revolvers firing particularly powerful cartridges).

The first generation of cartridge revolvers were converted caplock designs. In many of these (especially those that were converted long after manufacture), the pin on which the cylinder revolved was removed, and the cylinder was taken from the gun for loading. Later models used a loading gate at the rear of the cylinder that allowed one cartridge at a time to be inserted for loading, while a rod under the barrel could be pressed rearward to eject the fired case. Most revolvers using this method of loading are single-action revolvers.

Oddly enough, the loading gate on the original Colt designs (and copied by nearly all single-action revolvers since) is on the right side, which may favor left-handed users; with the revolver held in the proper grip for shooting in the left hand, the cartridges can easily be ejected and loaded with the right. This was done because these pistols were intended for use with cavalry, and it was intended that the revolver and the reins would be held in the left hand while the right hand was free to eject and load the cartridges.

Since the cylinder in these revolvers is firmly attached at the front and rear of the frame, and since the frame is typically full thickness all the way around, fixed-cylinder revolvers are inherently strong designs. Because of this, many modern large-caliber hunting revolvers tend to be based on the fixed-cylinder design. Fixed-cylinder revolvers can fire the strongest and most powerful cartridges, but at the price of being the slowest to load and unload and they cannot use speedloaders or moon clips for loading, as only one chamber is exposed at a time to the loading gate.

The next method used for loading and unloading cartridge revolvers was the top break design. In a top-break revolver, the frame is hinged at the bottom front of the cylinder. Releasing the lock and pushing the barrel down brings the cylinder up, which exposes the rear of the cylinder for reloading. In most top-break revolvers, the act of pivoting the barrel and cylinder operates an extractor that pushes the cartridges in the chambers back far enough that they will fall free, or can be removed easily. Fresh rounds are then inserted into the cylinder, either one at a time or all at once with either a speedloader or a moon clip. The barrel and cylinder are then rotated back and locked in place, and the revolver is ready to fire. Since the frame is in two parts, held together by a latch on the top rear of the cylinder, top-break revolvers can not handle high pressure or "magnum"-type rounds. Top-break designs are largely extinct in the world of firearms, but are still commonly found in airguns.