Both Pyrodex and 777 have been mentioned here. Remember that when guns are rated by the manufacturer for 150gr, they mean black powder or pyrodex...NOT 777. 777 is about 15% more powerful, so the charge needs to be reduced (max about 130gr).
I haven't shot pellets in awhile, but I remember getting similar results with 100gr pellets vs 100gr powder. 777 is different; knowing that people will get confused between the powders, the "50 gr" 777 pellets are not equivalent to 50gr loose. This is to prevent people from dropping 3 down the barrel like they did with pyrodex pellets and over-charging the gun.