You still don't get it.
The FLOUR is making the impact, not the bottle. Also, the flour isn't attached to the back of the bottle. No matter how easy it is to crush the bottle, the flour is still going to fly forward. Even if you just shot a bag of flour, it would be doing the same thing.
As long as you try to use the energy of the cannon shot to disperse the flour, it's going to be dangerous.
Also, you should reread pbmann's posts. He's been telling you all along how LOW-POWERED his cannon, not powerful.