The Mosin-Nagant is one of the longest serving rifles the world has ever seen. First entering service in 1891 as the standard infantry rifle of the Russian Empire, it is still in service with insurgent groups, the Finnish military (somewhat modified as the 7.62 Tkiv 85, but undeniably a variant), and possibly some Russian law enforcement. There have been an estimated 36,000,000 produced by Russia/USSR alone, and while not all countries' production can really be counted towards the total (Finland continues to use already produced parts for its guns, including receivers taken from captured Russian weapons), millions more were produced in various countries. With this long history of use, its no surprise that Mosins of one variant of another have been used in every major war since its adoption. At a minimum (Soviet/Warsaw Pact M44s and Chinese Type 53s) any Mosin Nagant is a piece of Cold War history, issued to Eastern bloc armies, militias, and affiliated insurgencies and terrorist organizations, while others may have seen service in WWI, the Russian Civil War, the Finnish Civil war, the Winter War, WWII, the Continuation War, the Korean War, the Vietnam War, and/or one of the various internal conflicts of the Warsaw Pact. While still in use in Afghanistan and Iraq, you aren't likely to run into a rifle used in either conflict due to US soldiers no longer being allowed to bring back "trophies".
Mosins are used by civilians for a variety of purposes, from target shooting to hunting. The 7.62x54R round can be had for as little as $0.20 a round, making it one of the most affordable rifles to shoot, and with proper ammunition, the Mosin can be used to take down even the largest game in the US.
As a defensive rifle, Mosins generally suffer compared to more modern battle rifles such as the FN FAL, AR15, and AK, due to its limited magazine capacity and lower rate of fire.
Even so, multiple Mosins may be had for the cost of a single more modern rifle, and in a really sticky situation the gun makes a very good war club or, with the addition of a bayonet, a spear, making it a good choice for outfitting a fixed defensive position, bug out bag, or even a platoon of friends on a budget.
The Mosin's issues are more than made up for by the cost of the gun and its ammunition. At current prices, even the most cash strapped can afford to own and regularly shoot a Mosin. Stockpiling ammunition is well within the realm of possibility for most individuals, meaning that, with a little bit of prior planning, you can easily have enough ammunition to last for years.
While harshly criticized for accuracy, the Mosin is considerably more accurate than most give it credit for (as long as the gun is in good condition). The gentleman in the video is hitting steel at 944 yards, without a scope.
Every man should own at least one Mosin, the budget battle rifle.
Current prices range from $139 all the way up to ~$700 for a 91/30 PU sniper. "New" production Mosins are not currently available in the US (Molot produces "new" Mosins for the hunting market in Europe by modifying existing Soviet Era guns).