Current Sierra and Hornady data show 24.8 grains with 240s as maximum. In their Third Edition, Hornady went as high as 25.0 grains as maximum. Hodgdon's maximum is 24.0 grains with the H110/W296 twins. I couldn't find data hotter than 25.0 grains, but it may exhist in older sources. Personally, 23 grains (Hodgdon's start load) is plenty for me. If I want lower velocities, there are far better powders like 2400, H4227 or BlueDot.
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