what has been said is right on. 777 50gr pellets have been "dumbed down" or weakened to be equal to 50gr Pyro pellets. They are NOT equal to 50gr of loose 777 - more like about 40gr of loose 777..

I've tested the 3fg 777 in my 50cal Knight and here is what I found:
1) there really is no need to reduce the charge when going to the 3fg 777---------- both the 2 & 3fg are cut so small that there is almost no difference in how they shoot.........just that the very slighty smaller grains of the 3fg tend to shoot a little more consistently. Hodgdon's load data shows less than a 100fps difference between Identical loads of 2fg & 3fg with the same bullets.
2) Lighter bullets tend to shoot WORSE groups with the 3fg powder and heavier bullets shoot better with it. In my .50cal Knight, 300gr bullets are the cutoff point - up to & including 300gr bullets get 2fg point, while my 350s and 400s get 3fg powder.
3) PowerBelts do not like 3fg 777 no matter WHAT weight you shoot. The powercheck cups won't release from the bullet and the accuracy goes to hell. I guess there is enough pressure difference with the 3fg to cause this problem.

But this is ONLY with 777 powder. For Black Powder or Pyro RS vs Pyro "P" - you BEST reduce your 3fg charges a good 10 or 15% below known 2fg charges - otherwise you might find yourself in a heap of trouble.