STW is an abreviation for (Shooting Times Westerner).
Originally created as a wildcat cartridge by necking down a standard 8mm magnum case to 7mm.
It is essentially a ballistic clone to the 7x300 Weatherby, which is an older wildcat and uses the 300 WBY case necked to 7mm.
For a period of time it became a factory offering by at least one gun company.
Id say the Rem 721/700 actions were the choice for the early guns, although any good action including the Savage 110 can be used.
The Rem 7mm ultramag has slightly higher velocity potential than both of the others mentioned.