The sst is a good choice 777 pellets may not be.
If you must shoot pellets get some Pyrodex pellets in both 30 and 50 grains and try it this way wiping between every shot. 3x30 grain, 2 x 50 grain, 4x30grain, 2x 50grain +1xx30. do not exceed your guns max recommended powder load which I believe to be 3x 50 grain which does not usually show much accuracy.
On the other hand if you wish to get the best your gun will do get a u-view TC volume measure and a can of Blackhorn start at 90 grains and go up 10 grains for each group mark you targets and go up to 120 grains when you know which load is shooting the tightest group constantly try 5 grains on each end [up and down] to find your long range load.