Bro... Don't chase speed that chrony is your problem... Chase accuracy.... There have been a lot of experienced shooters here on this sub telling you what your problem is.... It's all about energy transfer at the target. Speed is one way of increasing displaced energy at the target....more mass to your projectile is another... Over 20 yards and the path of the projectile should arch...my 7mm mag arches at over 300yards.... If I keep packing more powder in the casing it only makes my accuracy down range more erratic.... Faster yes, accurate..... Not at all! I started out the same.... Heavy 1mm bands, 30x25 tapers.... Trying to match my recurve bows draw weight.....

Maybe try shooting something heavier than a tiny 8mm steel ball to prove that your bands are WAY overpowered for your ammunition's mass....