8-32 Flange Nut / Weld Nut
Flange nuts, Weld Nut, or Binding Nut can be used to help secure your railbuttons to your rocket.
8-32 threads for 1010 Railbuttons
When installing the flange nuts inside your rocket, depending on the thickness of the body tube, a portion of the flange nut may protrude. That should sink into your Rail B