It is an 8mm Remington Magnum case necked down to 6.5.
It is an extremely fast caliber in the 6.5. It has about 95gr of usable case capacity. I personally think it is to overboar of a caliber. I have a 7STW and love it. It shoots 168gr Bergers at 3150fps eaily out of a 26" barrel. I would do a 7 STW over a 6.5 STW, and would look into what twist you would need to shoot the 180gr bullets.
I have one and it shoots very well. It is one that you work up your load, zero it, and use it for hunting purposes. If you plan on shooting a lot this is not your rifle. It is top of the line in 6.5. Any more powder capacity with that small a bore and barrel life is not good. I have five 7 STW's and they are a better overall hunting rifle because of all the good bullet choices with much better BC's than 6.5mm and longer barrel life. Easy to load the 6.5 version. Just get 7STW brass and neck it to 6.5. This one and the 257 STW got popular several years ago until the barrels burned out and they got rebarreled into something else. Both are fantastic deer-antelope rifles out to 700 or so yards. I still have a couple of each and love to shoot them. They are not a lot faster though than the 257 wby and the 264 winchester. Matter of fact my 257 wby is faster than my 257 STW.