I'm getting different info from various people, and from various different firearms, so I don't have a load I consider correct yet, but the average load seems to be a duplex load.
10gr black powder in a tea bag above the primer, followed by a 550gr-620gr load rolled in silk or similar material cylinder to take up volume.
550gr charge with 1550gr 20mm vulcan inert training projectile apparently net approx. 2700fps, while a 620gr charge with the same projectile will hit 3000+fps. Should be getting up towards the 30,000 ft-lb - 35,000 ft-lb range.
Here is a photo (not mine) of some pulled charges from vintage Finnish wartime production ammunition. They salvaged what material they could is my understanding.
Again, this is information from the internet and I don't recommend trusting it without much more research.