The way I make 500/450 No. 2 Musket brass from Starline Sharps 50/90 brass:
1. Anneal Sharps 50/90 brass from shoulder area to mouth of case
2. Full length size one piece of brass in 500/450 no. 2 Musket sizing die (special sizing die not
required)
3. Inspect sized case to determine if case mouth is crumpled due to interference by short die.
3.a. If mouth of case was not crumpled during resizing proceed to step 4
3.b. If mouth of case was crumpled during resizing then trim remaining cases so that they are
not crumpled during full length sizing but still slightly longer than desired case length
for 500/450 No. 2 Musket. Then proceed to step 4.
4. Resize cases
5. Trim cases to desired length of finished cartridge case

Please note that the base of the Sharps 50/90 case is a bit undersize for this conversion. I have used a couple hundred cases made this way and loaded with black powder equivalent to factory loads or modest amounts of some smokeless powders and I have never suffered any negative issues. I have read of several other shooters making the 500/450 No. 2 Musket from Sharps 50/90 brass with no reported failures due to the undersised base. If you choose to follow my method you are however on your own.