Don't know if you would consider this a back up gun.
I carry IWB. Sometimes when weather dictates I have a coat. When it is cold I button it up.
Those times a J frame or Kahr goes in my front right hand coat pocket as a second gun.
Not really back up more like quick access gun.
I would not go lower then 9mm or 380. Only becasue that is what I have, nothing against 380, I just have guns that fit the niche in a larger caliber.