Mike,
As I've mentioned in other posts (and in various articles and books) there is no firm relationship between different powders of very similar burn-rates, because any powder will vary slightly in burn-rate from lot to lot. IMR4350 and H4350 are a good example. When H4350 appeared, many handloaders firmly stated it was slower burning than IMR4350, and much of Hodgdon's data backed that up.
But one of the interesting side-effects of companies posting their latest data on the Internet is a clearer look at how powder lots vary. Since it was first introduced, the latest data for H4350 has been both "slower"
and "faster" than the latest data for IMR4350. This isn't a mystery, just the way powders vary over time.
Another little piece of evidence came with my first two lots of IMR4451. The first was a couple pounds sent to me by Hodgdon to try out. I liked it so well I ordered an 8-pound jug off the Internet--and discovered its burn-rate was slightly faster, not surprising since it was over a year after my first sample showed up, and the jug was a new lot. After a little more testing, I eventually mixed what remained of my first two pounds in with the jug--and not surprisingly, the mixed IMR4451 was a little slower than the jug's initial burn-rate.
What I understand from powder companies is that's exactly how new batches of powder are mixed to result in a reasonably consistent burn-rate for handloading powders. They keep some slower and faster lots of that powder on hand, so they can "adjust" the burn-rate of a new manufacturing lot.
On the other hand, many ammo companies use unmixed powders, adjusting the charge according to how the new lot burns. But they're usually boxcar loads of powder, and have pressure-testing equipment.
