Two pellets of 777 are roughly equal to 100gr of Pyrodex or 100gr of ? Black powder about like GOEX FF. or 85 gr of loose 777 or 78 gr of Blackhorn.
If you check it on a chronograph and there is the velocity difference that you think then the 777 has either drawn moisture or set on the shelf to long.
Never have had those problems with Black powder, Pyrodex or Blackhorn.
I did have them with APP and have heard of them with 777.