I was talking with my tire guy the other day about this. On my big truck, I will not accept anything other than a spin balance. If a tire shop cant do it, then they loose a sale. I had to get some new steer tires the other day and since our normal shop was slammed, I had to go to Action tire. They lost the sale cause they wanted to powder balance my steers. I had just had my Pete aligned and you're talking $600 (11R/24.5 Bridgestone) each for tires. The powder will not properly balance and can cause improper wear and vibration, which affects front end parts. If you've ever seen the repair bill for those size king-pins you would agree! Anyway, I brought up the subject of BB's and powder etc., with my regular guys. They told me that metal BB's WILL destroy the inside of your tire and can erode the rim itself over time. (kinda like sand blasting) They weren't certain how the plastic airsoft type stuff would work, but would be better than metal for sure. Of course, what we do with our tires is a complete different story than a big truck, but tires are tires. DON'T use metal BB's! A blowout in a 35" ish tire at highway speeds could be fatal. There's a lot of space between the rim and pavement, and a whole bunch could go wrong fast. Just my .02 is all.