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Serbia in the 1890s was not a large or wealthy kingdom, and they had no domestic arms manufacturing capacity - but they did appreciate a good rifle and a good cartridge. The Serbian Army was armed with their M1880 rifle, which was a slightly improved Mauser 1871 single shot design, chambered a the Serbian-designed 10.15mm cartridge. By the mid 1890s this was seriously obsolete, and Serbia began to look for ways to replace it.
In 1898 they were able to secure a loan from the Austro-Hungarian Empire to purchase new rifles, and they chose the 1895 pattern Mauser, in 7x57mm. However, Mauser was at that time at full capacity making rifles for Turkey, and had to hand the Serbian order off to DWM. By the end of 1900 the full order of 90,000 rifles (plus a stockpile of ammunition) had been delivered, and most of the Serbian fist Ban forces were back on technological par with the rest of the world. These Model 1899 rifles would be followed by the 1899/07 and the Model 1910, both of which were basically the same action in the same caliber.
In World War One, the Serbs with their Mausers would make a good first showing against (ironically) the Austro-Hungarians forces, but they did not have the stamina or resources to repel a second major offensive in 1915, when the Serbian Army was routed and forced to evacuate to the Greek island of Corfu. Most of their arms were lost in the process, and the Serbian Mausers would see no more organized use in the Great War (the army was re-equipped with French rifles instead).