You don’t actually need the adapter to fire the APNCR ammo. Doing so only comes with a slight loss in penetration.
Having it on means no HE ammo, likely no APHE either. Because squeezing a HE round to a smaller calibre would break the chamber and cause an accidental detonation.
So point 2 means Gaijin would have to remove or disable those rounds when the modification is enabled.