Typically, a bullet is propelled by the contained deflagration of an explosive compound (originally black powder, later cordite, and now nitrocellulose), although other means such as compressed air are used in air rifles, which are popular for vermin control, hunting small game, and casual shooting ("plinking").
In most armed forces the term "gun" is incorrect when referring to a rifle; in the military, the word "gun" means a cannon (artillery). Furthermore, in many works of fiction a rifle refers to any weapon that has a stock and is shouldered before firing, even if the weapon is not rifled or does not fire solid projectiles.
Originally, rifles were sharpshooter weapons, while the regular infantry made use of the greater firepower of massed muskets, which fired round musket balls of calibers up to 19 mm (0.75 inch). Benjamin Robins, an English mathematician, realized that an elongated bullet would retain the mass and kinetic force of a musket ball, but would slice through the air with greater ease. The innovative work of Robins and others would take until the end of the 18th century to gain acceptance.
By the mid-19th century, however, manufacturing had advanced sufficiently that the musket was replaced by a range of rifles—generally single-shot, breech-loading—designed for aimed, discretionary fire by individual soldiers. Then, as now, rifles had a stock, either fixed or folding, to be braced against the shoulder when firing. Early military rifles, such as the Baker rifle were shorter than the day's muskets, and usually the weapon of a marksman. Until the early 20th century rifles tended to be very long—an 1890 Martini-Henry was almost 2 m (6 ft) in length with a fixed bayonet. The demand for more compact weapons for cavalrymen led to the carbine, or shortened rifle.
The performance of early muskets was sufficient for the styles of warfare at the time, whereby soldiers tended to stand in long, stationary lines and fire at the opposing forces. Aiming and accuracy were not necessary to hit an opponent.
The origins of rifling are difficult to trace, but some of the earliest practical experiments seem to have occurred in Europe during the fifteenth century. Archers had long realized that a twist added to the tail feathers of their arrows gave them greater accuracy. Early muskets produced large quantities of smoke and soot, which had to be cleaned from the action and bore of the musket frequently; either through the action of repeated bore scrubbing, or a deliberate attempt to create "soot grooves" that would allow for more shots to be fired from the firearm might also have led to a perceived increase in accuracy, although no one knows for sure. True rifling dates from the mid-15th century, although the precision required for its effective manufacture kept it out of the hands of infantrymen for another three and a half centuries, when it largely replaced the unrifled musket as the primary infantry weapon. In the transitional nineteenth century, the term "rifled musket" was used to indicate the novel weapon. During the Napoleonic Wars the British army created several experimental units known as "Rifles", armed with the Baker rifle. These Rifle Regiments were deployed as skirmishers during the Peninsular war in Spain and Portugal, and were more effective than skirmishers armed with muskets due to their accuracy and long range.
Gradually, rifles appeared with cylindrical barrels cut with helical grooves, the surfaces between the grooves being called "lands". The innovation shortly preceded the mass adoption of breech-loading weapons, as it was not practical to push an overbore bullet down through a rifled barrel, only to then (try to) fire it back out. The dirt and grime from prior shots was pushed down ahead of a tight bullet or ball (which may have been a loose fit in the clean barrel before the first shot), and, of course, loading was far more difficult, as the lead had to be deformed to go down in the first place, reducing the accuracy due to deformation. Several systems were tried to deal with the problem, usually by resorting to an under-bore bullet that expanded upon firing.
The original muzzle-loading rifle, with a closely fitting ball to take the rifling grooves, was loaded with difficulty, particularly when foul, and for this reason was not generally used for military purposes. Even with the advent of rifling the bullet itself didn't change, but was wrapped in a greased, cloth patch to grip the rifling grooves.
The first half of the nineteenth century saw a distinct change in the shape and function of the bullet. In 1826 Delirque, a French infantry officer, invented a breech with abrupt shoulders on which a spherical bullet was rammed down until it caught the rifling grooves. Delirque's method, however, deformed the bullet and was inaccurate.
Another important area of development was the way that cartridges were stored and used in the weapon. The Spencer repeating rifle was a breech-loading manually operated lever action rifle, that was adopted by the United States. Over 20,000 were used during the Civil War. It marked the first adoption of a removable magazine-fed infantry rifle by any country. The design was completed by Christopher Spencer in 1860. It used copper rimfire cartridges stored in a removable seven round tube magazine, enabling the rounds to be fired one after another. When the magazine was empty, it could be exchanged for another.
As the bullet enters the barrel, it inserts itself into the rifling, a process that gradually wears down the barrel, and also causes the barrel to heat up more rapidly. Therefore, some machine-guns are equipped with quick-change barrels that can be swapped every few thousand rounds, or in earlier designs, were water-cooled. Unlike older carbon steel barrels, which were limited to around 1,000 shots before the extreme heat caused accuracy to fade, modern stainless steel barrels for target rifles are much more resistant to wear, allowing many thousands of rounds to be fired before accuracy drops. (Many shotguns and small arms have chrome-lined barrels to reduce wear and enhance corrosion resistance. This is rare on rifles designed for extreme accuracy, as the plating process is difficult and liable to reduce the effect of the rifling.) Modern ammunition has hardened leadcore with a softer outer cladding or jacket, typically of an alloy of copper and nickel - cupro-nickel. Some ammunition is even coated with molybdenum-disulfide to further reduce internal friction - the so-called 'moly-coated' bullet.
The increased velocity meant that new problems arrived, and so bullets went from being soft lead to harder lead, then to copper jacketed, in order to better engage the spiraled grooves without "stripping" them in the same way that a screw or bolt thread would be stripped if subjected to extreme forces.
The advent of massed, rapid firepower and of the machine gun and the rifled artillery piece was so quick as to outstrip the development of any way to attack a trench defended by riflemen and machine gunners. The carnage of World War I was perhaps the greatest vindication and vilification of the rifle as a military weapon. By World War II, military thought was turning elsewhere, towards more compact weapons.
By contrast, civilian rifle design has not significantly advanced since the early part of the 20th century. Modern hunting rifles have fiberglass and carbon fiber stocks and more advanced recoil pads, but are fundamentally the same as infantry rifles from 1910. Many modern sniper rifles can trace their ancestry back for well over a century, and the Russian 7.62 x 54 mm cartridge, as used in the front-line Dragunov Sniper Rifle (SVD), dates from 1891.
During and after WWII it became accepted that most infantry engagements occur at ranges of less than 300 m; the range and power of the large rifles was "overkill"; and the weapons were heavier than the ideal. This led to Germany's development of the 7.92 x 33 mm Kurz (short) round, the Karabiner 98, the MKb-42, and ultimately, the assault rifle. Today, an infantryman's rifle is optimised for ranges of 300 m or less, and soldiers are trained to deliver individual rounds or bursts of fire within these distances. The application of accurate, long-range fire is the domain of the sniper in warfare, and of enthusiastic target shooters in peacetime. The modern sniper rifle is usually capable of accuracy better than 0.3 mrad (1 arcminute).
In recent decades, large-caliber anti-materiel rifles, typically firing 12.7 mm and 20 mm caliber cartridges, have been developed. The US Barrett M82A1 is probably the best-known such rifle. These weapons are typically used to strike critical, vulnerable targets such as computerized command and control vehicles, radio trucks, radar antennae, vehicle engine blocks and the jet engines of enemy aircraft. Anti-materiel rifles can be used against human targets, but the much higher weight of rifle and ammunition, and the massive recoil and muzzle blast, usually make them less than practical for such use. The Barrett M82 is credited with a maximum effective range of 1800 m (1.1 mile); and it was with a .50BMG caliber McMillan TAC-50 rifle that Canadian Master Corporal Rob Furlong made the longest recorded confirmed sniper kill in history, when he shot a Taliban insurgent at a range of 2,430 meters (1.51 miles) in Afghanistan during Operation Anaconda in 2002.
Currently, rifles are the most common firearm in general use for hunting purposes (with the exception of bird hunting where shotguns are favored). Use in competition is also very common, and includes Olympic events. Rifles derived from military designs have always been popular with civilian shooters, and today semi-automatic versions of modern military rifles such as the AR-15 or AK-47 have become very popular in the United States for target shooting and sporting purposes despite the 1994 Federal Assault Weapons Ban which restricted their manufacture to some degree until 2004, when it expired. In other countries however, the ownership of these types of rifles may be restricted by law, and so single-shot or bolt-action rifles are common instead.