By the year 2291; the Anti-Fascists were still utilizing the STG-44 and T&H STG-72 rifles against the Fascirians. Although those rifles were good; they needed a new weapon with a new recoil system to improve upon the STG-44. Later on a weapons shipment was attacked over Fascira by Odin-Class Attackers; this led to a small freighter to almost escape and then crash landed after being hit by an attacker. The ship crash landed; spewing hundreds of weapons; including the AK-47 and AK-74 assault rifles.
Anti-Fascists that discovered the AK-47 and even AK-74, gave them an idea to create a new rifle and that was the STG-91. The next battle between the Fascirians and the Anti-Fascists was the STG-91's first test. The weapon killed 200 Fascirian troop; leading to a retreat and the weapon gained fame.
Since it's creation in 2291; the weapon was kept in service and even given to other cultures across the galaxies. Some members of the Intergalactic Alliance of Freedom in fact utilize these rifles as well.