It could be very possible, there is a certain amount of time of which the bullet is scripted to die at, probably in milliseconds after hitting, or on the rare occasion, seconds. I look at it as this: Each Avatar is equal to one prim. When spraying bullets at a prim wall, you get rebounds which in my perspective could kill one avatar, bounce of him then hit and kill another avatar. Thus if this would be an Xbox related achievement, it'd have to earn a good 150 GP.