416 Rem Mag , 358 STA, 7mm STW, 257 STW, 375 H&H AI, 300 Ackly Improved, 300 & 338 Jarrett and countless other wildcats since the 8Mag came about in 1978, It's not simply the 375 H&H case necked down, Remington designers actually applied wildcat techniques and improved the case by pushing the shoulder up, enlarging the top of body from the ridiculously diving taper of the 375 H&H at .448" to .486" allowing up to 5-6 gr more case capacity, making the 8mm Rem Mag case much more attractive than the outdated skinny look of the 375 H&H ...
Roy Weathery shortened his Wby cases to 2.825" from the standard H&H/Rem max case length of 2.850" and went with a longer neck pushing the shoulder down and created his signature radiused shoulder design .... BUT improved the top of body/shoulder junction diameter to .492" in order to maximize case capacity ....
The 8mm RM, 300 @ 340 Wby are within +- 1 grain in case capacity difference, brass make will change capacity somewhat as well
Back in the early 90's I exclusively hunted Alaska moose, brown & black bear with a 300 Win Mag & 8mm Rem Mag and have absolutely noticed the difference in immediate killing effectiveness of the 8mm Mag over the 300 Win Mag .. on parr with the 340 Weatherby ....
Peterson is now making top notch 300 Wby brass and I've got a bunch of it for my 300 Wby & necking it up for my 340 Wby, 8mm Rem Mag & my brothers 375 H&H , this brass should breathe new life into the 8mm Rem Mag, 350 STA, 7mm STW etc ...
I'm definitely resurrecting my 8mm Rem Mag as it's my all time favorite cartridge
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