So 100 gr (V) of 777 is roughly equal to 78 gr weight. 120 gr of 777 (V) would be about 94 gr weighted.
Since 120 by volume (94 gr weighed) is the max recommended load of 777 in a standard muzzleloader, I would not use over that.
When I used 777, I never used over 100 gr volume with a 300 gr...