Sir Galahad
Rocketeers use FFFFg black powder for ejection charges, not motor making.
The ejection charges expel the recovery hardware from the rocket during flight.
Making black powder motors is difficult and dangerous,
requiring high compression of the black powder, and the
addition of binders, if I'm not mistaken.
It is not sanctioned by either of the national rocketry organizations.
The largest commercial black powder motor is the 'E9' made by Estes.
Larger black powder motors are a tricky proposition, prone to failure.
That's why there are none above an 'E' in size.
Commercial and most experimental high power hobby rocket motors
are made with non-explosive ammonium perchlorate composite
propellant because it is a very stable, safe material. It cannot explode.
RocketMan
NAR Level 2 High Power Certified