there is something called a hop up which causes the bb to spin backward, causing it to fly up
rifling to make a bb spin would cause insane amounts of inaccuracy
This. The hop-up adds a certain level of the right kind of spin to make the bb go out, then up, then back down instead of going out and falling to the ground at a short distance. With rifling there would be no predicting where the bb would go because it would put too much spin on it. You would shoot it in one spot and it could land right there or it could land 5 feet off to the right. lol