The two are similar, but not the same - yet*. H4350 is less sensitive to temperature changes than is IMR4350. From the 2007 Hodgdon Manual, 24" .25-06, max loads with 100-grain bullets:
50.0/H4350 = 3155 fps - 51,500 cup
53.5/IMR4350 = 3290 fps - 52,900 cup
* Since Hodgdon owns IMR, they may eliminate one of the 4350 powders - just makes business sense.
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