Change in IMR 4064?

Discussion in 'Reloading' started by Matt_G, Jul 30, 2009.

  1. Matt_G

    Matt_G Well-Known Member

    Messages:
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    Joined:
    Feb 7, 2002
    Does anyone here know if there has been a significant change in the burn rate of IMR 4064 in the past few years?

    I ask because I am noticing a big difference in 7mm-08 loads in my two Sierra manuals.
    Specifically:

    4th edition (50th anniversary)
    7mm-08
    168 SMK
    Max load of IMR 4064 is 42.0 grains

    5th edition
    7mm-08
    168 SMK
    Max load of IMR 4064 is 39.2 grains

    That is a pretty big drop in the max load.
    So what is going on here?
    Did the powder change that much?
    Different testing methodology?
    Or is the 39.2 grain max load a "lawyer" load?