The SA80 rifle is originally from the Kenner G.I. Joe Action Force line and was also released with the Imperial Stormtrooper in Hoth Battle Gear in the YPS comic (#510) as gimmick. It seems like they ran out of Imperial Hoth rifles and paired some of the Snowtrooper figures with the next best thing: SA80 rifles from the G.I Joe line.
There are 4 main colour variations. Only the black one was released (partially) with the Imperial Stormtrooper in Hoth Battle Gear on the YPS comic.