The Standard Infantry Rifle, often referred to as SIR, was the standard issue weapon of the United States military following its reconstruction in the aftermath of the Battle of Yonkers.
The Standard Infantry rifle was chambered to fire 5.56 NATO rounds. The origins of the SIR remain unknown. There are various theories ranging from being a derivative of the AK47 to a stripped down version of the XM8.
Due to composite materials being difficult to manufacture, the SIR was made primarily of wood and was described as looking similar to a World War II-era weapon. The weapon had high recoil and only had semi-automatic fire to prevent soldiers from wasting ammunition if they panicked when facing Zombies. Due to these qualities, it is very likely that the SIR is an M14 rifle that has been re-chambered to use 5.56 mm instead of 7.62 mm NATO rounds.
The SIR first saw extensive action at the Battle of Hope. Afterwards during the Road to New York it remained the primary weapon when facing zombies. The weapon was very popular with soldiers due to its accuracy and reliability. In addition it was easy to modify and could be converted to a carbine or sniper rifle easily. They also came equipped with a bayonet for close quarters, a feature that was used fairly frequently. As a result of these features soldiers affectionately referred to the weapons as "Sir."