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After the conclusion of the First World War in 1918, the M1917 would be modified into the M1919. The M1919 uses the same short recoil mechanism as the M1917, but replaces the water-cooled barrel with an air-cooled barrel covered in a shroud. This dramatically cut down on the weapon's weight, with the M1919 coming in at 31 pounds. The change to air-cooling instead of water-cooling came with some complications, however. When it was decided to try to lighten the gun and make it air-cooled, the M1917's 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 (this kind of unintended weapon discharge is known as a 'cook-off'). With each further shot heating the barrel up 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. M1919 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. They were also taught 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, in order to delay heating. Most other air-cooled machine gun designs were fired in this 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).
The M1919 originally fired the .30 caliber M1906 (.30-06) ball cartridge, and later the .30 caliber M2 ball cartridge, contained in a woven cloth belt and feeding from left to right. A metal ammo link was later adopted, forming a "disintegrating" belt. Operation of the M1919 was much the same as it had been for the M1917. Loading was accomplished by inserting the pull tab on the ammunition belt from the left side of the gun 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, 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, the gun performed the sequence of extracting the spent round from the chamber and extracting the following round from the belt as the bolt came rearward, the fresh round ejecting the spent one when the bolt was to the rear and the fresh round cycled in front of the bolt, then on the forward stroke chambering the next round to be fired, advancing the belt, and engaging the next round in preparation for loading. Once the bolt closed, the firing pin dropped and the round was fired, and the sequence was repeated (at a rate of roughly ten cycles per second) until the trigger was released or the ammunition belt was exhausted. Except for the M1919A6, all M1919 variants had to be mounted on a tripod or other type of mount to be used effectively. The tripod used by infantry allowed traverse and elevation. To aim the gun along its vertical axis, the adjustment screw needed to be operated. This allowed the point of aim to be moved upwards or downwards, with free traverse to either side, allowing the gunner to set an elevation and sweep a wide band of fire across it by simply moving the gun from side to side. There was no need to control barrel climb or keep careful track of the fall of shots to make sure the fire was falling at the proper range. The gun was aimed using a small folding post at the front end of the receiver and a rear aperture sight on a sliding leaf with range graduations from 200 to 1,800 meters in 200 meter increments. When folded down, the aperture formed a notch that could be used to fire the gun immediately without flipping up the leaf. The rear sight also had windage adjustment with a dial on the right side.
However, by and large the most common variant of the series was the M1919A4. Production blueprints of the new variant were complete in late 1936, and production soon followed. The driving force behind the development of this variant was the suffering reliability of the 18-inch barrel of previous versions, which did not produce enough recoil to cycle the action reliably. The bull barrel was made much thicker and was lengthened to 24 inches like the M1917. Various other small adjustments to the design were made, such as moving the front sight from the barrel jacket to the receiver, which made it easier to mount the gun on vehicles. The design of the barrel jacket was changed to include circular holes instead of long slits of earlier models, and a recoil booster in the muzzle end improved reliability. The recoil buffer assembly was also a new addition to the design between A3 and A4 development, designed to reduce the impact of the bolt hitting the backplate.
