The M1907/12 heavy machine gun was the standard mounted MG of the Austro-Hungarian Empire during the First World War, and saw sporadic use clear through the end of WWII. The design is unusual among heavy machine guns because it is actually an unlocked, delayed blowback system. A combination of a heavy recoil spring and significant mechanical disadvantage is used to retard the breech to the point that extraction pressure is low enough to be safe (and extraction is aided by the use of a cartridge oiler). These are definitely underappreciated guns today, being one of the most compact and simple guns of their type.
For more information on the design, and to download electronic copies of a couple original manuals, check out my Vault page on the 1907/12.