I have also heard of people using BBs as well, and I wasn't sure why they did that either. So how exactly does it self balance? Seems like the water or BBs would always be at the part of the tire that is making contact with the ground.
Centrifical force, it's like taking a bucket with water in it and swinging it around our head. Notice that at low speeds the water doesn't move the same thing happens in the tire. The weight goes to the spot that requirs balance. I run 14oz of airsoft BBs in my XMLs and hear them move only when I slow down below 1mph or stop.