I started using 4451 pretty early on, at first getting three pounds for testing, then liking it enough to buy an 8-pounder maybe 6-8 months later.
There was a little difference in burn-rate between the two batches, not uncommon in new powders, enough to result in about 50 fps difference in .257 Roberts loads with 100-grain bullets. I eventually mixed the two batches, resulting in velocities about 10-15 fps less than with the unmixed 8-pounder.
This mixed 4451 turned out to be slightly faster than what's left of my last batch of H4350, but not much. A good example is my Kilimanjaro 7x57, where I loaded 46.0 grains of H4350 with 160-grain Sierra GameKings for right around 2700 fps. Had to drop the charge with 4451 to 45.5 grains to match that.
Have had similar results so far in other rounds, so obviously the two powders are so close in burn-rate that different lots would probably overlap.
