A Russian 7.62×54mmR bolt action rifle.
It entered service in 1892 with the Russian military and was primarily used in the Russo-Japanese War, World War I, and World War II. The Mosin-Nagant features a straight/horizontal bolt handle (except on the sniper variant, which had a bent bolt handle), slot-type gun sling mounts (except for the Finnish variants), and a partially visible 5-round internal magazine in front of the trigger.
This tag implicates bolt_action and rifle (learn more).