First question, is it 2F or 3F?
3F is for modern muzzle loaders only.
2F can be used in cartridges, shot shells and modern muzzle loaders.
Hodgdon does not recommend that any grade of 777 be used in antique firearms.
Load it to just shy of the base of the bullet. Hodgdon does not recommend compressing 777.
yeah, yeah yeah... just like Hellgate I have used it wrong too. I was in a place I couldn't get BP for a while and used APP and 777 flown in for us (funny story about what is considered dangerous and they won't transport on military contracted flights...). I just loaded it like I do Real powder.
I also don't care what grain size of powder I load for Cowboy shooting, I shoot FFFg, FFg, Fg and probably floor sweepings. I mainly shoot reenactment powders when I can get them (so I'm cheap...). I have shot Fg from cap and ball revolvers and even 4F and 4F mixed with 1F. As you can see I'm a "snob" (NOT) when it comes to shooting at targets 10 to 15 yards away from revolvers, rifles and shotguns.
Now saying all of that and sounding smug, I will admit I just paid $67.99/lb for 4 lbs of IMR 4064 last week which is only enough for just 699 loaded rounds (unless I spill some), with tax, shipping and hazmat that is over $0.47 per round! I will spend money when it counts but as I said, cowboy action ranges unless you are in a PRS or some sort of long range side match just shoot what you can get.
You can always have space in a case (horrors!) even with BP. John Kort found a bunch of 19th century shells loaded in partially filled cases. I'm not sure where the "use a full case or seat the bullet against the powder" came from. Smokeless powder generates a
LOT more pressure a
LOT more quickly (I can show you the pressure ramp rate charts).
Hellgate's suggested 13 grains sounds good and he is RIGHT, 777 loads are still very low pressure loads. The manufacturers have to warn us that eating the powder can be harmful to our health and Prop. 65 will warn you that you should not attempt to reproduce after using a substance as dangerous as 777.
All joking aside, firearms are serious things, they can and will kill you or others if mishandled. But, BP and BP substitutes are not some mystical art, they are in fact easier to use than smokeless powder.
~Mako