How many steel balls are there in 0.85 rubber band?

0.85 rubber belt with 8 to 10 mm steel ball. According to the relevant public information, the 0.85 flat rubber band has a middle distance of 80 cm to 90 cm, and the initial launching speed can reach about 70 meters per second with 6 mm steel balls, which is considered as the best speed. If 8 mm steel balls are used, they may be overweight and prone to parabola, while steel balls smaller than 6 mm are too light and easy to float.