Dave,
I've seen both good and bad on the net about 777.
You won't hear bad from me. Chris Hodgdon told me as far as he knows this is the first African animal taken with 777.
Rifle was a Westley Richard Improvement by Francotte , Martini in 577/450. The load was made up with Bertram brass, Winchester large rifle primers, 65 grains by weight of 777 FFG powder, a filler mix consisting of 5 grains each of 777 FFG and Cream of Wheat, Two discs of waxed paper as a lube barrier separated the charge from the fully lubed (including hollow base) .460/460 grain hollow base bullet, and plenty of crimp.
I don't know about using pellets in cartridges, but I do like the loose stuff.