It's probably safe to load the black powder equivalent of 25 grains of 777 in your brass frame gun which would be about 21 grains.
Many folks do and their guns didn't suffer any immediate damage.
Others have reported shooting the equivalent of 30 grains often enough (minus 15% for 777) which is about 25 grains of 777 but that's starting to push it.
There are other weaker powders to use but you purchased 777 which is simply the most potent powder. Try it and then tweak the load as you see fit. Just make sure that the ball is seated all of the way down on top of the powder.
777 burns pretty clean and shooting it should still be enjoyable.
If the brass frame ever stretches and loosens up then the parts alone are still worth a good portion of the purchase price: the barrel, the grip frame, the internals.
Indentations will appear on the inside of the recoil shield behind the cylinder to serve as a warning that the brass frame is getting battered.
Enjoy your gun for what it is or exchange it for a steel frame gun before shooting it if you still can.
