BE SURE your reamer is to the Griffin & Howe specs and the dies are too.
The real 400 Whelen as chambered by G&H was not just a necked up 06'.
Fred Zeglin's Book on Wildcat cartridges has a whole chapter by Mile Petrov on what the 400 Whelen should be. There are many, many incorrect reamers out there.
I publshed the book for Fred:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=237839867

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That's my 400 on the left. Works well.

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Sad thing is that Barnes stopped making the 350 gr .411 X