The M1919 Browning is a .30 caliber medium machine gun that was widely used during the 20th century, especially during World War II, the Korean War, and the Vietnam War. The M1919 saw service as a light infantry, coaxial, mounted, aircraft, and anti-aircraft machine gun by the U.S and many other countries.

The M1919 was an air-cooled development of the standard U.S. machine gun of World War I, the John M. Browning-designed water-cooled M1917. The emergence of general-purpose machine guns in the 1950s pushed the M1919 into secondary roles in many cases, especially after the arrival of the M60 in US Army service. The United States Navy also converted many to 7.62 mm NATO and designated them Mk 21 Mod 0; they were commonly used on riverine craft in the 1960s and 1970s in Vietnam. Many NATO countries also converted their examples to 7.62 mm caliber, and these remained in service well into the 1990s, as well as up to the present day in some countries.

The M1919 originally fired the .30 cal M1906 (.30-06) ball cartridge, and later the .30 caliber M2 ball cartridge, contained in a woven cloth belt, feeding from left to right. A metal M1 link was later adopted, forming a "disintegrating" belt.

Loading was accomplished by inserting the pull tab on the ammunition belt from the left side of the gun—either metal links or metal tab on cloth belts—until the feeding pawl at the entrance of the feed way engaged the first round in the belt and held it in place. The cocking handle was then pulled back with the right hand, palm facing up (to protect the thumb from injury if the weapon fired unexpectedly, which could happen if the barrel was very hot), and then released. This advanced the first round of the belt in front of the bolt for the extractor/ejector on the bolt to grab the first cartridge. The cocking handle was then pulled and released a second time. This caused the extractor to remove the first cartridge from the belt and chamber it (load it into the barrel ready to fire). As the bolt slid forward into battery, the extractor engaged the next round on the now-advanced belt resting in the feedway, preparing to draw it from the belt in the next firing cycle.

Every time the gun fired a shot, it performed this sequence: the bolt came rearward, extracting the spent round from the chamber and pulling the next round from the belt so that the fresh round ejected the spent one. As the bolt came forward, it chambered the fresh round, advanced the belt, and engaged the following round in preparation for loading. Once the bolt closed, the firing pin dropped, and the round was fired.

The sequence was repeated roughly ten cycles per second until the trigger was released or the ammunition belt was exhausted.

The gun's original design was as a water-cooled machine gun (see the M1917 Browning machine gun). When it was decided to try to lighten the gun and make it air-cooled, its design as a closed bolt weapon created a potentially dangerous situation. If the gun was very hot from prolonged firing, the cartridge ready to be fired could be resting in a red-hot barrel, causing the propellant in the cartridge to heat up to the point that it would ignite and fire the cartridge on its own (a cook-off). With each further shot heating the barrel even more, the gun would continue to fire uncontrollably until the ammunition ran out, since depressing the trigger was not what was causing the gun to fire (although rarely as full rate automatic fire; it takes time for heat to soak into a cartridge, so usually it would manifest as a series of unexpected random discharges, the frequency increasing with the temperature of the barrel). Gunners were taught to cock the gun with the palm facing up, so that in the event of a cook-off, their thumb would not be dislocated by the charging handle, and to seize the ammunition belt and pull to prevent it from feeding, if the gun ever started an uncontrollable cycle of cooking off. Gunners were trained to manage the barrel heat by firing in controlled bursts of three to five rounds, to delay heating. Most other air-cooled machine gun designs were fired in the same way, even those featuring quick-change barrels, and which fired from an open bolt, two features that make air-cooled machine guns capable of somewhat more sustained fire, both features that the M1919 design lacked.

When the gun was ready to fire, a round would be in the chamber and the bolt and barrel group would be locked together, with the locking block at the rear of the bolt. When the rear of the trigger was pivoted upwards by the operator, the front of the trigger tipped downward, pulling the sear out of engagement with the spring-loaded firing pin, allowing it to move forward and strike the primer of the cartridge.