The 7mm was supposed to be released at the same time as the 270 but shortly before release it was discovered that it was possible to chamber and fire a 270 WSM round in a 7mm WSM rifle. The 7mm was delayed until a minor redesign prevented that.
Thats backwards, a 7WSM could be fitted into a 270 WSM chamber, the neck of the 270 chamber was/is large enough to allow an unexpanded 7WSM neck into the chamber and the bullet would squeeze into the throat, to prevent this they moved the shoulder ahead on the 7WSM by about .038" so the bolt wouldn't close on it if it were ever inserted in a 270WSM chamber. You can fire a 270 WSM in the 7WSM and 300WSM chamber and the case will expand to fill the larger chamber and the bullet will just rattle loosely down the barrel without causing any damage to the gun or shooter.
