Went on a school trip to Fort Ontario in Oswego, New York and it looks like they added a new piece to the weapons collection.

(Stats for M-1919-A4 as it was the most produced and pictured)
Put into service in 1919 (no shit) the weapon was a redesign of the water-cooled M1917 that fired the .30-06 Springfield cartridge (nicknamed the Thirty Ought Six) and was produced up to 1945.

(Wikipedia )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. (Wikipedia)