8mm rem mag necked to 30 caliber. Been around since the introduction of the 8mm rem mag when it was necked to everything. Basically a 300 wby mag without the radius shoulder. Both are the same performance.
It goes by many names, 30-8mm Rem Mag, 30 Super, 30 STW, 300 STW. Also almost every reamer is designed by the builder and will vary from gunsmith to gunsmith.
It is basically an 8mm Rem Mag necked down to what ever, 30, 7mm, 6,5mm.
Also some are "improved", less body taper with 30, 35, or 40 degree shoulder.
I have one that I use for 1,000 yd paper punching.