Pyrodex in the pan will ignite from the sparks off of the frizzen maybe once in 15 trys. When it does, it won't "flash" or "explode", it will just burn a little faster than smokeless powder.
A flintlock needs a flash of the pan powder to send the flame thru the barrels vent (touch hole) to ignite the main powder charge in the barrel.
Pyrodex and the other synthetic powders all have a ignition temperature of 800 degrees F or higher.
This means the feeble flash from the pan won't heat it high enough to ignite.
If you use real black powder in the pan to get the flash the gun needs, it still will have a very hard time getting the main charge of Pyrodex to ignite. Using real black powder in the pan and Pyrodex in the barrel will get the gun to fire maybe once every 5 times.
Real black powder has a ignition temperature of about 450 degrees F which makes it fairly easy to ignite. That is why it works so well in both the pan and in the barrel.
Some folks who have some Pyrodex or other synthetic powders on hand and want to shoot them in a flintlock will save their black powder for priming the pan and to put maybe 5-10 grains (by volume) down the barrel and then load the Pyrodex on top of it.
This does work because the pan flash is good and the powder at the vent is real black powder. When the black powder in the barrel ignites it will fire the Pyrodex reliably.
zonie