The real balancing beads are tiny ceramic things. You can put them in the tire through the valve stem using a DA sander to vibrate the stem and let them flow through faster.
AirSoft BB's work in big tires, but you have to seat the bead with them int the tires. Some wrap them in paper towel and let the towel just disintegrate in the tire.
Do NOT use metal BB's, they will rust. For your 31's, just buy some balancing beads.
https://www.youtube.com/watch?v=ubyZNqgw40k