I've used IMR 4350 and H4350 with 180 gr bullets. Any data for IMR 4451, since it's sometimes easier to find?
Well, it's right next to IMR 4350 on the Hodgdon burn rate chart.
This is one where I would definitely start low and work up, stopping well short of "max". A U.S. Krag rifle is one that I would never hot rod.
That said, the late Michael Petrov conducted a destruction test on a U.S. Krag once by incrementally increasing the powder charge of Bullseye powder. It wasn't until he got to an unholy amount of Bullseye - 20+ grains if memory serves - that the action finally hand grenaded. Note also that he did it with the locking lug ground entirely off the bolt, relying solely on the safety lug to lock things up.
I use 4451 in the 30/40 but in high pressure loadings in modern actions. I would expect it to work quite well with 180's at around the 42-44 grain level in a Krag. I like the powder. It's a good choice for heavy bullet loads in the 22/250 and for heavy bullet loads(190+) in the 308. GD
My load with either IMR or H 4350 is 46 gr, and both shoot well. But, not always easy to find.
So far I've found 4451 produces just about exactly the same performance as H4350 in every cartridge I've tried it in, probably around 10 total, getting just about the same velocity with either the same charge, or at most a half-grain difference in rounds with the capacity of the Krag--at least with my present manufacturing lots of both powders.
You're welcome!
Have also found that the "universal" 46.0 grain 4350 maximum with 180-grain bullets is, of course, very mild. Have used 48.0 grains of H4350 in several original Krags with zero problems, mostly because it shoots more accurately than 46.0 grains. In fact it's the charge that "regulates" perfectly in my over-under .30-40 double. (It also happens to be my standard powder charge with 180's in 303 British rifles, which isn't surprising since the .30-40 was essentially a longer-necked copy of the .303.)
I found that in my rifle's 20"barrel, 46.0 gr of H4350 and a Hornady 180 gr BTSP chronos about the same as Winchester 180 gr factory loads.