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The 280/284 etc BORE diameter is .277 and the 270 is BORE diameter of .270 So that means 7.035mm is closer then 6.858mm

Nah .277 is only a 270 sluggard. and 10thou to small to be a 7mm :pacman:

Yeah but was made to counter the 223 during trials for the AR15

It's actually Winchester's fault they released the 264 magnum in around 1956 then did the 300 and 338 on the same case therefore Remington using the same case for a 7mm in 1960(?) called it the...

It comes from the original Weatherby line of cases I think late forties

The design of the case. The belt reinforces the base of the case to run higher pressures without blowing the case where it comes out of the chamber.