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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.
A crucial tactical mistake at the Battle of Yonkers was arming soldiers with fully automatic weapons such as assault rifles and "SAWs"(Squad automatic weapon) which gives a unit a high-rate of automatic fire to blanket an area with suppressive fire which is ineffective against zombies as they have no sense of self-preservation and therefore cannot be suppressed or forced into cover by gunfire.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 failed to take into account that the only way to kill a zombie is to render extreme trauma to the brain, and the overwhelming gunfire only resulted in creating swarms of legless zombies which were just as deadly but smaller targets. The defeat at Yonkers paved way to an Anti-zombie doctrine,which involves firing a concentrated volley of shots aimed at the head in a line or column formations, sometimes for hours at stretch. This resulted in the need of weapon as accurate as a designated marksman rifle that would not jam even after firing hundreds of rounds in succession that will foul most rifles 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/AR-15 rifle series. The SIR was designed to be semi-automatic only, forcing soldiers to fire one shot at a time. Training doctrine also mandates that Combatants take slow and controlled shots,at a steady advance. This lessens combat stress as combatants are taught to fight and advance at a slower pace than conventional combat and at the same time conserving 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 converted from a medium-ranged assault rifle, to a sniper rifle or a carbine for CQB(Close quarters battle)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.