Balisticaly the 7mm-300Wby is the same as the 7mmSTW , the Wby case has a tiny bit more capacity but the soft Norma brass won't let you take advantage of it ,you will be able to run the STW up to the same speed with a little more pressure though.
If you gonna go through the hassel then I do a 7mm STW Ackley improved , not alot will be gained from the improvement except case life. You can also shoot factory loaded 7mmSTW ammo out of a proberly cut Ackey chamber with little loss in speed or accuracy.
The 7mm-378Wby is way overbore capacity , you would only see any real benifit with very heavy bullet out of a very long barrel and the barrel life would be very short.
Necking down to 7mm and Ackley improving a 300 win mag case has been a though of mine for a while , I'm pretty sure that you would get the same maybe a we bit more case capacity as the STW case but it would be a little shorter and you would have the option to use the awsome lapua brass if you don't mind paying $1.50 a case , but 100 cases would last the life of the barrel.
then theirs always the 7mmRUM
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