I see one problem using that cutter, enlarges the entire hole and the edge comes very close to the oring on the brass piece. Unless FX or Huma moved that oring groove out from there backward towards the probe...I did my mod back 5 months ago I am not aware of a recent change.
I had a carbide ball or bullnose cutter OD 6.8 mm (shank is 3mm or 1/8 a dremel size). The OD fits nicely inside the M8 threaded hole.
The shank allows about 1.5 mm each side Left/Right offset inside the threaded hole comfortably. The end result is a slot, let say 7x9.8 mm. The slot direction is perpendicular to the barrel hole axis.