The service rifle (also known as standard-issue rifle) of a given army or armed force is that which it issues as standard to its soldiers. In modern forces, this is typically a highly versatile and rugged assault rifle, battle rifle, carbine, or designated marksman rifle suitable for use in nearly all theatres and environments. Service rifles are also often selected for their upgradability (e.g., the addition of underslung grenade launchers, sights, torches, laser sights, etc.)
Although certain weapons issued to special forces units are rarely considered "service weapons" in the truest sense, certain specialist rifles and submachine guns are categorized as such if issued as per standing operating procedures upon entering special environments or scenarios. These may include urban warfare (FIBUA/MOUT) and jungle warfare environments.
Originally, rifles used in combat were not standard-issue weapons like the service rifles of today. Rifles were for specialist marksmen only, whilst the ordinary infantry were issued less accurate smoothbore muskets which had a higher rate of fire, with bore diameters as high as 19 mm, or 0.75 inch. By the middle of the 19th century, however, rifles were becoming more and more common on the battlefield, with muskets being phased out. Originally, these combat rifles were single-shot muzzle-loading weapons, but as technology advanced through the 18th and 19th centuries, so too did the technique of loading rounds. First, breech-loading firearms, like the Prussian Needle gun of the mid-19th century came to prominence, which then evolved into repeating weapons, such as the bolt-action Mosin-Nagant rifle used by Imperial Russia in the late 19th and early 20th centuries and the Soviet Union in World War II. By this time almost all prominent armies in the world had some sort of standard service rifle.
During the Second World War, there was yet another leap forward in rifle design which was to influence service rifles even today. That is, the use of a fired cartridge's gas emissions to automatically rechamber rounds into the breech once a bullet had been fired, as well as expelling the old cartridge. These "battle rifles", as they were called, usually fired a "full-sized" (as opposed to an intermediate) rifle cartridge, such as the .30-06 Springfield or .303 British. Another type of commonly issued rifle which was to become well known during this time was the assault rifle, a (usually) fully automatic rifle firing a lighter "intermediate" cartridge, as opposed to the full-sized cartridges used by battle rifles. The first of these was the Sturmgewehr 44, used by Nazi Germany in the later stages of the Second World War. The StG44 was not issued in large numbers, and was never adopted as Germany's service rifle. However, this weapon was to serve as the precursor to other assault rifles such as the Soviet AK-47, the American M-16/70, the Belgian FN FAL, the German G3 and the Swiss Sturmgewehr 57, which today supersede battle rifles as the service rifle of choice for militaries the world over.