That'll work!
Which is kinda what major powder companies do after producing a new lot. They save some of the previous lots that were faster or slower-burning than the ideal they were, uh, "aiming" for. They test the new batch, then if it's not within 2-3% of the burn-rate desired, they mix in some of the older batches to adjust the burn-rate.
This obviously is easier to do when the powder's been in production for a while, so there are more previous batches. When Hodgdon started offering the IMR Endurons a while ago (primarily due to a shortage in the supply of the Australian-made Extremes), they sent me some 1-2 pound samples of various powders to try. One of them was IMR4451, which was designed to approximate H4350. I was impressed enough to order an 8-pounder from a website, but it resulted in around an 80 fps difference in the same loads. So I mixed the first batch into the 8-pounder, and it resulted in velocities closer to their published data.
Was going to post this cautionary story anyway John, but it's so germane to your post above I'll just put it here.
Back before Covid, I bought a pound of IMR 4451, found it to be everything you were saying it was, so followed up with an 8# jug. Just recently did some load work for a friend using .243W Nosler brass, the 95 gr BT, and your load from the "Little Book" of 42 gr 4451, which gave just under 3100 FPS from the 1 pound lot. Used up the last of the 1# in the process and had to go to the 8#. Did a little rework up ladder with the new lot to the old 42 grain charge. Was stunned to see only 2900 FPS with the new lot from the 8# jug. That's by far the biggest lot-to-lot variation (200 FPS, with everything else about the loads identical) I've ever encountered. One more data point to always back off and re-check charges when going to a new lot of even the SAME powder, much less the substitutions being described in this thread - this happened to get slower, but it could just as easily been on the faster side and dangerous.
Cheers,
Rex
