Many small arms are "single shot" firearms: i.e., each time a cartridge is fired, the operator must manually re-cock the firearm and load another cartridge. The classic single-barreled shotgun is a good example. A firearm that can load multiple cartridges as the firearm is re-cocked is considered a "repeating firearm" or simply a "repeater". The lever-action rifle of Old West fame, a pheasant-hunter's pump shotgun, and a police sniper's bolt-action rifle are good examples of repeating firearms. A firearm that automatically re-cocks and reloads the next round with each trigger pull is considered a semiautomatic firearm. An automatic (or "fully automatic") firearm is one that automatically re-cocks, reloads, and fires as long as the trigger is depressed. Many modern military firearms have a "selective-fire" option, which is a mechanical switch that allows the firearm be fired either in the semiautomatic or fully automatic mode. In the current M16A2 and M16A4 variants of the U.S.-made M16, continuous full-automatic fire is not possible, having been replaced by an automatic burst of three cartridges that makes full-automatic fire considerably more accurate.

The first "rapid firing" firearms were usually similar to the 19th century Gatling gun, which would fire cartridges from a magazine as fast as and as long as the operator turned a crank. Eventually, the "rapid" firing mechanism was perfected and miniaturized to the extent that either the recoil of the firearm or the gas pressure from firing could be used to operate it (which made the firing mechanisms truly "automatic"). Automatic rifles such as the M1918 Browning Automatic Rifle (the "BAR") were in common use by the military during the early part of the 20th century, and automatic rifles that fired handgun rounds, known as submachine guns, also appeared in this time.

Submachine guns (such as the well-known Thompson gun) were originally about the size of carbines. Because they fire pistol ammunition, they have limited long-range use, but in close combat can spray bullets in a deadly and controllable manner due to the light recoil of the pistol ammunition. They are also extremely inexpensive and simple to build in time of war, enabling a nation to quickly arm its military. In the latter half of the 20th century, submachine guns were being miniaturized to the point of being only slightly larger than some large handguns. The most widely used submachine gun at the end of the 20th century was the Heckler & Koch MP5. The MP5 is actually designated as a "machine pistol" by Heckler & Koch (MP5 stands for Machine Pistol 5), although some reserve this designation for even smaller submachine guns such as the MAC-10, which are about the size and shape of pistols.

Nazi Germany brought the world's attention to what eventually became the class of firearm most widely adopted by the military: the so-called assault rifle (see Sturmgewehr 44). An assault rifle is usually slightly smaller than a military rifle such as the M1 Garand, the M14 or the K98k. Generally, assault rifles have mechanisms that allow the user to select between single shots, bursts of shots, or automatic fire. Moreover, assault rifles tend to incorporate military characteristics that make them look menacing: bayonet lugs, flash suppressors, and large capacity ammunition magazines. Generally, these design features are trivial in nature, and the modern military assault rifle differs very little in function from a civilian hunting rifle. Universally, civilian versions of military assault rifles are strictly semiautomatic. The cartridge fired by these rifles is midway in power between a pistol cartridge and a high-power rifle round, which gives the soldier the close-in spray ability of a submachine gun with the more precision long-range shooting of a high-power rifle round. Soviet engineer Mikhail Kalashnikov quickly adapted the concept to the AK-47, which has become the world's most widely used assault rifle. In United States, John Garand, the inventor of the M1 Garand rifle used by the U.S. military during World War II, adapted the assault rifle design to produce the M14, which was used by the U.S. military until the 1960s. The significant recoil (hence inaccuracy) of the M14 when fired in full automatic mode was seen as a problem, however, and in the 1960s it was replaced by Eugene Stoner's AR-15, which also marked a switch from the high-powered but heavy .30-caliber rifle used by the U.S. military since before World War I to the much smaller but far lighter and light recoiling (and arguably more accurate) .223-caliber rifle. The military later designated the AR-15 to the "M16". The civilian version of the M16 continues to be known as the AR-15 and looks exactly like the military version, although it lacks the mechanism that permits fully automatic fire.

Modern designs call for compact weapons retaining firepower. The bullpup design, by mounting the ammunition magazine rear of the trigger system, unifies the accuracy and firepower of the traditional assault rifle with the compact size of the submachine gun (though traditional submachine guns are still used because of the ballistic properties of the handgun ammunition that they use); examples are the French FAMAS or the British SA80.

New ammunition design shapes are the future of firearms. Recently, smaller but powerful ammunition types have been introduced, as to allow personal defence weapons to penetrate ballistic armour. Such designs are the basis for the Five-seveN and the FN P90. Caseless ammunition is another trend, though impeded by the large number of traditional firearms, which would make their introduction expensive (an example is the German Heckler & Koch G11). The flechette is yet another improvement over traditional ammunition, allowing for extreme penetration abilities and a very flat trajectory.