What is "mauser-koka"

In 1880, Serbian Major Kosta "Koka" Milovanović developed an updated version of the Mauser Model 1871, still single-shot, but chambered in its unique 10.15×63R caliber. It had unique additions in that it had a bolt guide (much like the M1870 Italian Vetterli) and the "progressive rifling" that was developed by Koka. The Kingdom of Serbia adopted the rifle in 1881. It was designated Serbian Model 1878/80, also known as Mauser-Koka, Mauser-Milovanović, and known in Serbian as Kokinka . The grooves reduced in diameter from breech to muzzle. The muzzle velocity of the Mauser-Milanović was . It saw first combat in the Serbo-Bulgarian War. Approximately 110,000 Mauser-Milovanović rifles entered the Serbian arsenal. It was further developed in 1907.