I'm sure you've observed the entrees in the LoadSwap database, all of which address what I'd call max. loads. My experience is with the 360gr. LBT bullets and I started at 22.5grs W296 and max'ed at 25grs with my 6" Taurus. The Cast Performance recommended starting load for the 454 335gr. LBT bullet is 25.0gr of W296. Considering the loads listed in LoadSwap, this would represent more than the recommended 3% max. powder reduction the powder manufacturer recommends. But on the other hand, the max. loads in LoadSwap appear to be Freedom Arms only type loads and on the high side. I'd suggest using the Cast Performance starting load and use Marshall's recommended process for determining max. loads for your particular revolver.
You touch on one of the more popular topics; that is, is H110 and W296 the same powder or very similar powder. To make a long story shorter, I've checked four different loading manuals and the H110 and W296 loads do not vary more than .5grs.. So no problem with interchangability. I remember one post and another web site that quoted an email response from Hodgdon that said they were the same.
But with that said, please be aware that each lot of powder has potential for varing results. Thus, another good reason to "work up" the load whenever you change lots of powder.
I've had lots vary to the extent of a full grain difference for the same velocity.
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