I earn about 85% of my living making flintlocks. I know a thing or 2 about them.
No black-powder substitute will flash in the pan and/or light the main charge.
You need REAL black powder.
You can (and should) buy 25 pounds and have it shipped to your door.
If you are worried about how to store it PM me and I can give you a few tips on how and where to do it safely.
Oh.....buy the way; You can use 4F for the priming, but there is no real need. I use 3F and even 2F to prime and I have no problems at all.
Here are a few links to look at if you like flintlocks and classic muzzleloaders.
http://s90.photobucket.com/albums/k2...erican%20guns/
http://s90.photobucket.com/albums/k2...German%20guns/
http://s90.photobucket.com/albums/k2...nglish%20guns/
http://s90.photobucket.com/albums/k2...ie%20Harrison/
http://s90.photobucket.com/albums/k255/szihn/pistols/