You will probably get a strange Pressure Spike for sure...
Would 80 grains be enough to blow the barrel ?
Who knows , But I would not risk it. Not with 30 grains anyway.
Many people who use Pyrodex or 777 as the main charge
in a Flintlock, sometimes put a few grains of fff black powder
down the bore first to ensure that the flash from the pan
touches off the main charge. This does not seem to cause
a problem. All I would recommend would be to tie it to a tree
the first time you pull the trigger, And pull it with a long string.