Hello Lefty:
That sounds like a load for whitetail. I've no experience with sabots or 777, but 90 grs. of stuff in a 50 cal. frontloader should be OK. 45 cal. balls been used for a long time for deer, so why not a bullet?
I'm using 80 gr. of 2f with a patched RB in my 50 cal TC Hawken, works for the Muzzleloader season here in Pa. It's also nice for trying to walk up on a maneatin groundhog in the summertime. A farmer gets a big smile when I show up with my flinchlock. Guess he figures I'm harmless.
How does your Firestorm work with round ball? What part of Pa., east or west?
Woodbutcher