You folks are way ahead of me re smokeless powder loads for ML. But I'm very familiar with "nitro-for-black loads in later 1800s BPE (Black Powder Express) rifles. 15-20 years ago, I read that 3031 can produce pressure excursions that are highly worrisome, and probably that was being said about 45 and 50 cal. loads. I have little doubt that this is why 4198 became the powder of choice for this purpose.
Now we've got more powders, more pressure testing, and more experience. But one crucial item needs to be forever noted: caliber. The smaller the bore, the easier powder builds pressure. So any favorable pressure data applicable to MLs needs to include what caliber was being tested (and exactly which primer).
The other major issue is powder burn-rate. Pounding a bullet down a ML barrel creates way more tension on the bullet than does the bullet neck-tension in a cartridge case. There is way more resistance to getting that tight bullet moving. Smokeless powder may therefore burn very differently and pressure will therefore vary widely, and not likely in a good direction. All the parameters of a given pressure chart need to be considered, and too often all the parameters are not there. Especially for the relatively new smokeless powder MLs. Just the fact that they are almost all 40-50 caliber (to best of my knowledge) makes such loading a whole new ballgame.
I also well remember being told by the Western Powders senior Ballistician to avoid 5744 for nitro-for-black loads in my 45 cal. rifle, even tho it is the broad-use equivalent of Unique, because of possible pressure excursions, and that the best thing to use in my original 45-90 '86 Winchester loading was therefore Blackhorn 209. Perhaps his caveat stemmed from the behavior of 5744 in larger caliber barrels.
Aloha, Ka'imiloa
