From Wikipedia: “The Mosin–Nagant (Russian: Винтовка Мосина) is a five-shot, bolt-action, internal magazine-fed, military rifle, developed by the Imperial Russian Army in 1882–91, and used by the armed forces of the Russian Empire, the Soviet Union and various other nations.”
The version of this rifle that we use in-game is based on the M1891/30, a later revision of the original gun put into service in 1930 and lasting until 1944 when it was officially replaced by the SKS. The Mosin continued to see regular use in armed conflict up through the Korean and Vietnam Wars, and can even be seen on modern day battlefields thanks to its incredibly rugged design.
Most of the weapons we use in DayZ Standalone deviate slightly from the real-life guns they are based upon. Most of the changes made for the game version concern ammunition and attachments, for the sake of sharing those items between weapons. That said, the Mosin we use in-game is accurately chambered for 7.62x54mmR rounds, which most models of this gun were designed to use (less popular variants include 7.62x53mmR, 7.92x57mm, and 8x50mmR).
Where the gun in DayZ differs most from its real-life doppelganger is in the simplification of its available attachments for the game. Given the massive number of rifles produced, homebrew modifications and aftermarket solutions for attaching optics and other useful parts are widespread — something that may make its way into the game eventually through the use of crafting workbenches.