My question would be; What is the point?
The way bump fire works is by allowing the firearm to bounce enough to fire the rifle with involuntary or finger movement. To me it would make much more sense to take aimed shots. I do not believe you can watch the front sight, and rear sight, while holding a rifle that is bouncing all over the place. Remember, Front Sight, Rear Sight, Trigger Control? You are giving up all three.
If you are ever in a defensive situation that required automatic weapons fire to defend yourself, you are probably in the wrong place at the wrong time, and then, you would still need to be able to control your bullet placement.
If you just want to spray an area with expensive ammo, go for it. If you feel a need to make a lot of noise, buy firecrackers.