1918 Mauser TuF rifle - Tank und Flieger. It was the first purpose-built anti-Tank weapon, used a 13.2mm (.52") semi-rimmed cartridge very similar to the later .50 BMG. Matter of fact, the Americans could not get the performance they wanted with the powders available, so they copied the German powder.
There was also a TuF machine-gun which the Germans kept in production, part by part, until about 1921. It was a more-or-less standard Maxim-system gun, weighed about the same as Ma Deuce. Only a single specimen exists, though. Idea was to up-gun existing fighter planes, use them as airborne tankbusters: Warthog 1919. War ended before they could be put into action. I REALLY want one!