Development of PP loads for the .44-77.

[beltfed]

About 25 grains of 4227 or 4759 or Maybe 5744 under your bullet may work?????
I know as you pointed out that you are dealing with a bottleneckcase that needs to "go a ways to blow out.
But I had a good result for my 40-72W to blow out the Thick Hornady 405 brass to fit my chamber:
I used 25 gr 3227 under a 400 gr snover bullet. While fireforming, it turned out to shoot pretty good , too.
I will say that the Hornady brass started out from the factory with a Good Annealing about 1/3 way down from the case mouth.
"Just trying to help"
arnie