Not sure if this is an action question or Barloc question forum wise but here goes. Arc sells 3 different versions of the Barloc from what I can see. Is the recoil lug (on the Nucleus) different depending on whether you want to use the Barloc?
The Nucleus Barloc has a tab on top that interfaces with the scope rail, acting as a recoil lug for the rail and to index the recoil lug.