8bore,
Just checked my records and I'm using 70 Gr IMR 4198, CCI mag primers and 17 Gr dacron filler under a 610 Gr NEI bullet in my 577 3". Getting 1775 FPS with excellent regulation and accuracy. I'm using the mag primers because I sometimes got hang fires in the 577 case using regular primers. I think this has to do with the larger diameter of the case, the mag primers solved the problem.
Good Luck.