Just last night on Mythbusters, they did this experiment with firing a gun straight up into the air. The determination was that the bullets return to earth a no more than terminal velocity. However, if the gun was even slightly off of straight up, then it did carry some energy from the bullets original charge. The trouble comes in when you try to get it straight up, and also one must figure in the wind. I know this is not exactly on the subject asked, but it was a very interesting experiment. As far as the paintball shots from the airplane, I would be concerned that the plan may not work. My thought would be that you would have to slow the airplane down to get any kind of shot off, which is not ideal, in that it will expose you to enemy fire from the anti-aircraft guns.
The other thought that comes to mind, is that paintball guns don't have a tremendous amount of range. One would have to increase the pressure on the gun to get some velocity, and thus range, and given the fact that you are already travelling a fair speed, the pressure exerted on the paintball woud rupture the ball, then you would end up flying into your own paint. (which doesn't look good when you get back to camp with your own color paint all over your plane)
As far as the flour, it is great fun, but be careful one misguided flour bomb was dropped at our airport years back, and ended up in the FBO hanger. Went right through the steel roof. Roof stll has a drip there during heavy rains. (luckily no one was injured in the mishap)
Just my thoughts