Going by the cartridge name one could assume the "360" refers to the "bore" size and not the "bullet" diameter but that is only an assumption. My info above suggesting .358" bullet is from COTW so no guarantee it's correct. Huvius your experience confirms there is some confusion there.

The neck size given for the 400/360 WR is .375" and for the 400/350 Rigby is .380" which suggests a .358" diameter bullet and only a .350" bore. Unless they used thin brass in the 360's

The 360 No.2 NE has a neck dia of .393" for it's .367" dia bullet. The 9.3x62 also has ~.390" neck for it's .366" dia bullet depending on the source.

PT&G has a reamer listed for a 400/360 Rimless but no info on dimensions.

A cartridge drawing showing correct dimensions would be nice. Does anybody have one?