Just a note on BP history.
BP is typically used in Muzzle loading rifles. 4F (FFFF) is used as a primer in the pan for Flint lock rifles. It needs to be very fine (small grains) for easy ignition from the flint sparks and burn fast through the touch hole into the barrel chamber to ignite the main charge.
Courser, 2F or 1F, BP is typical for the main change that propels the projectile.
We want a very fast burn rate so use 4F BP in ejection charges.
Hmmmmm..... "historically speaking"......
I absolutely agree that BP was used in muzzle-loading (ML) smooth-bore shoulder arms of all sizes and later rifles. But BP was also used in ML canons, ML pistols, and ML shotguns. And while it was used in Flintlocks, it was used much earlier in wheel-locks and in the even earlier fusil which became a generic word for gun, but originally a fusil was a very early ML gun that was ignited by the use of a fuse thru a touch hole like early cannons.
The mechanism for igniting the BP inside the barrel improved greatly over the years, from the ifrst simple touch holes fired with a match or fuse, to the mechanical fuse holder in the wheel-lock, to the flint-lock, to the percussion cap which was generally speaking, the last major innovation of the muzzle-loading era before the the invention of the rear/breech loading gun with a cartridge which held a pre-loaded percussion cap, the BP charge and the bullet inside a (usually) brass tube.
Most ML guns that I know of used 4F in the pan for later matchlocks/wheel-locks, and flintlocks because it was easy to ignite. The size of the BP inside the barrel was dependent mostly upon the diameter of the barrel. Most cannons used 1F. Most large bore shot guns used 2F. Most large smooth bore muskets like 60 cal plus used 2F. Most rifles in the 30 to 60 cal range use 3F. Pistols are another matter. I've seen originals loaded with both 3F and 4F. It depends on the pistol.
Often the size of the BP grain was limited by the quality of the metallurgy of the barrel itself. The larger grains of BP allowed for a slower build-up of pressure which kept the barrel from bursting from over pressure. Many old ML guns from the 1700's and early 1800's had proof marks stamped into the barrels by whatever government was legally certifying the barrel for use and export. The barrels were often "proofed" by firing them with a double charge of both BP and whatever was being shot out of the gun.
As to BP used in ejection charges, I used 3F for years without any problems because being one of those ML rifle guys, its what I had on hand. But these days I use 4F because I have it and I'm pretty sure that's what is recommended by the rocket motor folks for ejection charges. And if it works for motor ejection then it ought to work just fine for electronic motor ejection as well.
I suppose that's enough muzzle-loading history for now. I suppose that one could argue that most of our rockets are simply replicas of antique propulsion systems. I load my parachutes in the muzzle end of the rocket. I told the guy at the gun shop where I first bought some DuPont 4F that I needed it for my muzzle-loader. He didn't ask any more and I didn't offer any more. I must admit that it was just a bit of subterfuge on my part as some of the gun shops in those days would not sell BP to the "rocketry" community. I did have an actual muzzle-loader so I was telling the truth......mostly....... oh dear...... maybe I should just delete this whole post............
Well, I am a sinner and I guess here's proof for all to see, and oh dear its even Good Friday. Too late now........Fortunately I'm a forgiven sinner by that guy with the nail holes. And that was long before I was ever called the "Rocket Rev." Even after all these years, still only by His mercy.
Brad the "thankfully forgiven" "Rocket Rev.," Wilson