Done it.. Thompson/Center Hawken .50cal recoil was pretty heavy.
T/C makes a very strong muzzleloader and I WOULD NOT try this with Spanish made "Traditions" firearms etc. (Read Randy Wakeman's review for more info)
95 grains of Goex FFg and two .490 balls, touching of course. Shot her at fifty yards and was surprised to see they only struck the paper slightly lower than usual, and were 1.5" apart from each other.
The oldtimers I hunt with up north say back in the day this was "Loaded for Bear".
Now, I'm not saying anyone should try this- though here I am to tell the tale, so consider for a moment.. Is firing a 400 grain conical any different than two roundballs (touching) that weigh in at 178 grains a piece?
My 0.2c