The Mosin-Nagant is a sniper rifle, most notably used by The End. It is unusual due to the use of tranquilizer ammunition instead of lethal ammunition.
The Mosin-Nagant is a bolt-action rifle, the first rifle chambered for the 7.62 x 54mm Rimmed cartridge, that was first produced in Tsarist Russia in 1892. It was in mass service during the Russo-Japanese war in 1904, and later saw more action during both World Wars. While original models had no scope, it was first used as a sniper rifle during the Eastern Front of World War II; it kept this role until the 1960's, when it was finally replaced by the SVD.
Although The End could have easily used an SVD, he instead opted for a Mosin-Nagant Sniper Rifle, which illustrates his age and his apparent reluctance to kill. The End's rifle, apart from being chambered for tranquilizer rounds, also replaced the normal stock with a paratrooper's folding stock and a pistol grip. Naked Snake can wield the Mosin-Nagant by defeating The End by stamina depletion. It is a very useful weapon for no-kill runs.
In 2014, Old Snake obtained a Mosin-Nagant scoped rifle from Drebin and used it during his operations in the Middle East to terminate Liquid Ocelot. The rifle was still the same and acted as a tranquillizer weapon.