I looked at the T/C pic of the Firestorm again and I see how it works in theory. I still think that it lends itself to fouling and is over engineered in order to facilitate an unnecessary powder pellet mandate. It also removes the ability to use a breechface scraper You might need some sort of breech cleaning device such as a stiff brush to clean the edges of that inverted cone. That little groove seems very important and also very likely to fould quickly.
Also, if you ram the pellet down hard, you spear the pellet onto the spike of the cone which is then pressed down until it blocks the supposed flash circle which destroys the advantage of the design. Not so? Think it through. Took me about 3 minutes. On the other hand, I could be wrong.
Back to the loading proceedure, you should pick the flash hole to make sure that it is clear. In a traditional ML, you would feel dry powder granules inside the breech area. A very happy feeling to a flintlock shooter.
You do not want to put too much prime in the pan. A little goes a long way. I like 4f Goex, but have used 3f without a problem. Your flash hole should not have fouling or fresh powder in it. This would make a slow burning fuse whereas you want a quick flash through the air in the flash hole.
As to the use of 4f as a very small initial charge for the pellets. Cheap insurance in my view as long as you stay well under the manufacturers maximum total load. 4f is very potent and spikes quickly in the pressure it develops. You should look at the T/C or Hogdon literature here. Also, not all pan chargers are created equally. I have spouts in two sizes -- 1 grain and 3 grain. Some ML shooter "experts" used to recommend an initial charge of about 5 grains (if I recall correctly) of black powder to get the pyrodex to fire reliably in a flinter. That may have even been on the Pyrodex site, but their instructions link is down at the moment.
I started with Pyrodex and still have over half a can that is well over a decade old. I was a cap only shooter then, so the duplex loading was not something that I know from my own experience. I did see people try it as a range officer and it seemed to work okay. Pyrodex was not accurate for me and I seemed to have trouble keeping my bore from rusting after using it. I went to real BP and have not had trouble since.
I also do not like pellets. I view them as a "one lump or two?" situation when I want to tweak the powder load to just the right amount.
In any event, DO NOT LOAD with only 4f Goex. That is a very fine priming powder and too finely ground for use as a main charge. Yoy would be risking your safety and the gun if you did this. I would buy a can of Goex 3f as a main charge for your .50. (It is a .50 -- isn't it?)
YMHS,
CrackStock