All my Machbusters are puns off Dirty Harry. Mach My Day, Machnum Force, Sudden Mach. It remains to be seen whether the rockets or the bad puns run out first
18mm and 24mm Machbusters involve very small, lightweight rockets, which are too small to carry electronics or much tracking powder. The most powerful motors available in those sizes - D21 and F32 - barely get past Mach.
29mm Machbusters have a wider motor range that, in the high-power range, can get well past Mach. They can carry electronics that can reliably document the altitude and velocity, so it's possible to prove that it made it to Mach. They can also carry tracking powder - and lots of it - to make it easier to find, and dual-deployment electronics so they'll stay in the field.
You can stick a D21 in a little tiny rocket and say it'll go to Mach, but you'll never see it again.