Above and beyond the ignition problem which can be solved but is more trouble than the results are worth Pyrodex work better in a side lock for me because 777 tends to blow the patch with great consistency.
If you feel you must use 777 start by getting some good wads slightly over sized to the bore then get a musket nipple and some good RWS musket caps.
The wad will protect the patch most of the time and the Musket cap will ignite 777 most of the time.
Number 11 caps ignite it most of the time but I could tel that every shot was a slight hang fire. Lee