The M1 Garand is probably the most historical weapon of the US Armed Forces as it played a fundamental role as the weapon of choice for many American troops during the war.
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The rifle was the result of the work of Canadian-born John Cantius Garand. Among a huge number of inventors at the time, he was the one who was quickly employed by the US because of his ingenuity in creating a mechanism that made use of propellant gases from a fired bullet to cycle the rifle.
This meant that finally, a rifle mechanism was created such that expelling a spent cartridge from a firearm and subsequently loading a fresh round became automatic. This was a huge advantage, because the gun no longer needed to be manually reloaded after every shot.
In war, a split second advantage makes a world of difference, especially when the enemy combatant is out to exterminate you as a threat.
From there, the M1 Garand has solidified its role in saving as well as claiming lives. In WW II, it was the most basic requisite arm provided to the ground troops. When bullets were no longer available, it was even light enough to sport a bayonet, to be used conveniently in close combat situations.
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The M1 Garand may not be the best rifle of all time. But it was because of its ingenuity that other players tried to outdo it, making the most dominant rifle today undoubtedly a result of what the M1 started.