Just before the turn of the 20th Century, the world's militaries were scrambling to move from black powder weapons to the new smokeless powder - more powerful, and cleaner burning. The Russian military was no exception. In 1891, a rifle design owing its existence two inventors - the Russian Sergei Mosin and the Belgian Leon Nagant - was adopted by the Tsar's military. How and why the two were both involved is a story unto itself.
The rifle was chambered for a new round. The bullet diameter (bore) was almost exactly three-tenths of an inch (.311) and the rifle was thus known by three names. It was called the 'Mosin Nagant' after its creators, and the 'Three Line Rifle' reflecting the use of 'line' as 1/10 inch in Russia - and the M1891 for its year of adoption.
The round, of course, was the 7.62x54R. The first number is the diameter of the projectile, in millimeters; the latter is the approximate length of the unloaded case (also in millimeters, although the specification calls for 53.72). The 'R' indicates that the cartridge is rimmed - that is, it has a protruding rim at the base that extends beyond the walls of the case itself. This makes extraction and seating of the bullet in the action more reliable, but designers were already turning away from it as it made magazine-fed designs more finicky. The Mosin Nagant, in fact, is prone to what is called rim lock - unless the interruptor and ejector are within tolerance, it is possible for the top round in the magazine to end up with its rim behind and interlocking with the rim of the round below it - and then it won't feed.
Despite this, the Mosin remained in service for nearly a century with various militaries. The 7.62x54R stayed with, and like its American military counterpart the .30-06 Springfield round, it went on to serve as a machine gun load despite the rim. The cartridges are quite similar in both dimensions and energy, although the 30-06 will edge out the 54R if given enough barrel. Both are used for hunting weapons, as well. Both were used for sniper weapons during the World Wars and after.
Today, in the U.S., this round is popular because surplus Mosin Nagant rifles are so readily and cheaply available. Over six million of the guns were made, and most were refurbished and then packed in cosmoline and stored by Soviet arsenals during and after World War 2. Although the USSR switched to semiautomatic rifles such as the SVT-40 and submachine guns such as the PPSh during the war, and to the famous AK-47 afterwards, the Mosins were often sent to communist guerrillas and regimes alike as foreign aid. In modern times, Russia and other alumni of the Eastern Bloc have discovered that American civilians like guns quite a bit and are happy to spend money on historic, rugged, and cheap firearms. Mosins are a favorite of the 'prepper' community for two reasons - first, they're cheap (until recently, you could often find them for $99, and $129 is still normal from importers). Second, and more relevant to this writeup, is that in addition to the rifle, those Eastern nations socked away millions and millions of rounds of 7.62x54R and, in some cases, continue to manufacture it for machine gun and sniper use. Best of all, that surplus ammo is packed as standard in crates of two 'spam cans' - 440 rounds, in 20-rd cardboard and paper boxes, sealed into an airtight metal tin with Cyrillic codes on them. Each crate is a wooden crate containing two of these cans, and can be had on the American market for between $150 and $200 a case, despite the massive inflation of ammunition prices during the Obama administration and subsequent pro-gun 'panics' about expected government interference in ammunition availability that never materialized - but which caused panic buying which did exactly that, and made ammunition manufacturers quite wealthy. The 7.62x54R remained fairly consistently available, although crates which had cost between $75 and $120 before the panic have nearly doubled in cost. Still, when 5.56x45 mm NATO standard ammunition can fetch up to $0.35-0.75 per round, even in qty 1000, $179 for 880 rounds remains a bargain.
It should be remembered that this is surplus military ammunition. The most important characteristics for the civilian enthusiast are that it is steel cased and Berdan primed, meaning that it's not possible to reload this ammunition. Some bullets are steel core, and some ranges will not permit steel core (magnetic) bullets on their ranges/targets. Still, it's cheap; and these days, there are civilian brass rounds available for sale, Boxer primed, which are quite reloadable. The bullet is a .311, the same projectile used by many other rifles such as the British Enfield family - so these bullets are widely available when reloading. The Boxer brass accepts standard large rifle primers.
The quality of this surplus ammunition varies widely. There are many sites on the internet which will help you read the manufacturing codes to help you identify the manufacturer (factory, site, etc.) which can be indexed to reviews of their quality. Most importers won't let you pick from their stock, so it's a bit of a crapshoot (haha) but sometimes you get lucky. In addition, it's quite possible to disassemble and reassemble this ammunition to better consistency than stock using home reloading gear - see haqiqat's Moscow Match for (much) more info.
A few of the weapons chambered for the 7.62x54R: