I just received an email from Hornady today. I shoot 100gr Pyrodex Pellets under a 250gr. SST from a TC Impact (26" bbl). Hornady stated:

"the velocity with a 100 gr charge is approx. 1890 fps. With a 150 yrd zero, the bullet will impact 2.9" high at 100 yds and 7.2" low at 200 yds. Thanks"

Of course your 777 should boost that velocity a bit. That's quite a bit more arc than they show for a 150 gr. charge, but if you know how high to aim who cares? As long as you hit consistently.