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This article uses an in-universe perspective.
The Standard Infantry Rifle (or SIR) was developed by the United States Government after they lost the Battle of Yonkers, retreated westward, and re-established order. It was used en masse at (and after) the Battle of Hope. This rifle (along with the M16 series rifles) are still being used by the US military after the Zombie War.
One of the many mistakes at the Battle of Yonkers was arming soldiers with semi-automatic or fully automatic weapons such as the "SAW" gun. This was part of the so-called "scythe-theory" developed by military commanders at the start of the epidemic, which proposed that spraying automatic fire at zombie-hordes would literally cut them in half. While this is exactly what happened, they utterly failed to take into account that the only way to kill a zombie is to destroy the brain, and the "scythe-theory" only resulted in creating swarms of legless zombies which were just as deadly but smaller targets. Further, this simply wasted hundreds of rounds of precious ammunition. Anti-zombie warfare requires firing a large number of accurate headshots from behind fixed positions, sometimes for hours at stretch. This resulted in the need for a designated marksman weapon that would not jam even after firing hundreds of rounds in succession, and this led to the design and development of the Standard Infantry Rifle.
Many elements of the weapon were borrowed from the AK-47 and the AR-15 (M-16), essentially combining the reliability and cost efficiency of the AK-47 with the accuracy of the M-16. A major point was that the SIR was designed without the ability to fire in Fully Automatic mode, forcing soldiers to fire one shot at a time. This way it was physically impossible for a soldier to panic and wildly fire in full-auto burst, using up his ammunition. The non-metal components of the SIR were usually made of wood, at least during the early years of the war, because it was difficult for the damaged survival-based economy of the Safe Zones to build artificial composite materials. The SIR also came with conversion kits which allowed the SIR to be quickly shifted from a mid-range rifle, to long-range sniper, or close-combat carbine. The front of the SIR was also equipped with an eight inch long flip-out bayonet, for close-quarters battle.
The SIR also typically fired the incendiary/explosive Cherry PIE rounds.