I’m really looking for some science on how the powder sizes react. Would a faster burning fff push my round ball out of the short barrel too fast to allow the rifling to get the spin up to speed?
Since BP burns on the surface of the granules (Pyrodex is chlorate enhanced BP, with a dextrin as a fuel source, rather than charcoal; because of this, it burns slower than BP), the smaller the granules, the faster it is consumed. This changes the pressure curve of the explosion in the barrel. For it to be efficient, your powder has to be consumed well before the bullet leaves the barrel (so the bullet gets the most acceleration from the powder). Another thing that has an effect on burn rate is the density and height-width ratio of the charge. Because finer powders burn faster, the breech pressures will be higher, but the time it takes to be consumed is less, so you can get away with a shorter barrel. Courser granulations are capable of a little higher velocity, due to the moment of force being longer, but you have to balance that with the afore mentioned considerations (small pebble powder in a 20 bore rifle with a 32" barrel works great, think cannon powder that has been sifted for size).
Stabilizing projectiles with rifling is a function of RPM. The faster you shove a projectile down the barrel, the faster it is spun. The issue you can run into (especially with projectiles with extremely limited bearing surface like a RB) is that the projectile can strip off of the rifling, and not be spun.
As far as why you are having accuracy issues, it can be many things. Your patching material may be too thin/too loose of a material, your projectile may be too undersized or oversized, your rifling may be too deep, it may be too shallow, you may have loading inconsistencies (especially if you are trying to compress the loading and are using volume to meter powder); and you might just be in between accuracy nodes for the rifle. My .50 RB rifle is a 26" 1:48 twist shallow groove (.005") rifled, and shoots just over 1.5 MOA with 100gr (weighed) of 3F and a paper patched ball. It's still good if I go down to 85gr, but then it goes to manure until I get down to 50gr. Likewise, If I go up to 110gr, accuracy starts to struggle, until I get up to 130gr. If I change the powder granulation or formulation, it shifts the nodes. I want to get some 2F and try it again, since when I tried 2F before, I was doing the whole "compressed load and volumetric powder measure" thing, and see if I can get a little more out of it.
