Originally Posted by Orvis
Does this mean then there there is a minimum safe load in a cap and ball revolver other then the minimum amount needed to safely push the ball out of the barrel?
Yes, in some old guns there is a powder chamber that the bullet/ball will not fit in. The powder must fill that chamber, and then some. Enough powder to propel the bullet out of the barrel, and enough so the bullet will seat completely on the powder.
The bullet helps control the explosion of black powder, with the air gap the powder explosion will accelerate it to much higher pressures than the steel can withstand, then it'll hit a proverbial roadblock causing everything to move - ball down the barrel and steel expanding from it's original shape. The term is often called "ringing the chamber/barrel".
Moderately compressed blackpowder loads are safer than "loose loads." With blackpowder cartridge pistols, you can fill the case up completely flush to the rim then seat a bullet on it compressing the powder - which is how it is supposed to be done.
With long rifle cases like the .45-70 and the longer buffalo cartridges, you need a 18" to 36" brass drop tube to drop the powder through to make sure there is little air in it. Then you seat a bullet, which often compresses the powder too.
If you use a drop tube on a smaller case like a pistol case you don't need to fill it to the rim, you can fill it to about 1/8" below the rim, sometimes less - but enough to have the bullet on the powder once seated.
Because of the need to have a full case of powder, and black powder's costs - it is way cheaper to shoot smokeless in the same cartridges(if the gun can handle smokeless). Smokeless can have some airspace.
40grs of Black Powder for a 250gr slug
16grs of BlueDot for a 250gr slug
Note, black powder is by volume and smokeless by weight - but the BP weight is still way more than the smokeless.