I don't see how a 7 mag would chamber in a 270. 7-08 would but a 7mag should be quite a bit fatter top to bottom, add in the belt and I can't see how the bolt would close. ???
If you are referring to the article I provided a link to it is because it was a 270 Weatherby, not a 270 Win which it would appear that you are thinking of.
"It was a very small mistake with very big consequences; he fired a 7mm Remington Magnum cartridge in a rifle chambered for .270 Weatherby."
drover