I actually wrote to Sierra about this and they responded.



Quote
Fotis,
I�m very sorry about this extended delay in answering your e-mail.
I hope I haven�t inconvenienced you.

I think our 300 grain SBT GameKing will be perfect for elk and moose even at the high velocities of your 378 Weatherby. The big bears will not be a problem either in that this bullet has a jacket thickness at the mouth of the jacket of .018� tapering quickly from .070�. I think you can use it with confidence.
While the jacket and core may separate it won�t happen until the bullet has expended the majority of its energy and is at the end of its penetration path.

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"Weatherby was too long so I nicknamed it "Bee""