welcome: to geybeard and good luck with your soon to be chosen whisper. Its my favorite plinker and suburban deer caliber now.
SSK Barrels shoot .223 brass turned into 300 whisper just fine. At least in mine they do. Also shoot just fine thru my Thompson Custom shop barrel and we all know Thompson Center was the only barrel maker that JD Jones allowed to License his 300 whisper so in practice, the chamber of a Thompson barrel and the SSK barrels are the same. The barrels have different twist rates though. I am getting my new 24 inch encore barrel from SSK this week. Since I have five hundred new 300 whisper brass made by Hoser here on greybeard,,,,,I sure hope they will work and am certain that they will.