It's the best of the modern black powder substitutes. I still prefer real black powder (Goex, mostly), but 777 is my choice if for some reason I can't use the real stuff.
777 meters very well, and looks and feels like black powder. It is less dense, meaning a given volume of 777 will weigh quite a bit less than an equivalent volume of black powder. That means more charges per pound, since you load 777 by volume, just as you do black powder.
777 also burns hotter, though -- about 15% hotter by most accounts. That makes it great for hunting rifles, but not so great if you're trying to achieve moderate velocities. To get the same velocity as black powder, you need to reduce the charge (by volume) about 15%. Thus, if you were loading 2.2 cc of black powder, you'd load just 1.9 cc of 777 for the same velocity.
As with black powder, you shouldn't leave an air space. Compression is not necessary, but light to moderate compression works well.
777 does not contain sulfur, so it lacks that rotten egg smell characteristic of real black powder. It also produces a lot less fouling, however, so you have to clean less often, and get less accuracy degradation than you do with black powder.
