My email to Winchester and their response:
"April 25, 2011
Sirs,
Are Hodgdon HP-38 and Winchester 231 the same powder?
That is, can the same reloading data for one be used for the other?
Thanks,
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HP-38 and Winchester 231 are exactly the same powder. They always have been the same powder. The only difference is the label on the bottle. Because they are the same powder, the data would interchange.
Mike Daly
Customer Service Manager
Hodgdon Family of Fine Propellants
Hodgdon Smokeless Powder
IMR Powder Company
Winchester Smokeless Propellants
GOEX Blackpowder"