Ive used CLR after seeing Cortina using it.
It works brilliantly without polishing/abraision like the Jb bore paste (which i also use). CLR and a bronze brush in a drill is my 'go to' for removing carbon in the neck and throat of the bore. Jb bore paste might polish that area too much judging by my experience of it and checking with a borescope the results.
Just like any chemical you have to use it correctly! As noted by Erik Cortina, you must flush it out thoroughly with alcohol or methylated spirits. This is easy, use the clr to remove the carbon, push a dry patch through, then two wet meths patches followed by a dry patch, job done. Remember that the bore with be very clean and prone to moisture so definitely lightly oil (i prefer 'crc longlife' - it way outlasts gunoils in a finer coating) to finish.
Welcome to the club @
andyanimal31 [emoji106]