I don't have anything to measure my NEF bore, but I use crush rib sabots and 300gr XTPS and also FPB's with great results like your number 8 target. don't forget to get the lowes number 60 O ring, I replace mine after 25 shots or so.

Semi , how tight is your forearm screw? Also, are the original scratches you said you had at the muzzle end of the barrel still there or did they polish out.

The MU plug is sweet for sure ,Busta did a bunch of testing on it and it shows. He is the guru on them , if you have questions I am sure he can help.
Steve