Slingshot ball dragging rubber band 2050 hanging 1842 hitting beads.

I have been playing the 2050 trailer 1842 for a long time. First, I have to adjust the length. 2050 is slightly longer than 1842 by one centimeter. Until the slingshot doesn't shake I have tried three kinds of steel balls: 9, 9.5, 10. I think the most suitable is 9.5, with straight trajectory and low power. 9 and 10 will do. 9 The following are not considered.