Interesting, I have the fat wrench and all the standard hex bits I have tried, worked. I've definitely used mine to torque Tikka action screws too. Are you finding that they're too short? Or the wrong diameter?
I bought a bunch in other sizes (Torx, hex, etc) that are Dewalt or Ace Hardware brand.
You will need to get another bit from whatever tool kit you use for your gun smithing. I don’t pay any attention to the size. I just grab one that fits properly. The fat wrench doesn’t have all of what I need for my Tikkas, that I know.