Pyrodex does NOT fire in a flint gun, nor does 777 or the rest of the powder substitutes. You're dealing with about a 300 degree difference in ignition demand (so I was told). I've even tried black in the pan, but no dice. I had to pull the load, and this was tried about 5 times with different ignition in the pan.
A note on cleaning: no matter what you use, you still have to work on the soap&water. If your rifle has what's called a hooked breech, it's a LOT easier to clean. Track of the Wolf has a device that screws into the nipple threads, and works
really well.
https://www.trackofthewolf.com/List/Item.aspx/578/1
Unfortunately though, my Kentucky doesn't have that hooked breech, so it's a lot more difficult to get really clean around the pan and touch hole. Most of those types of rifles have pinned barrels.