Milling Media Geometry

For making black powder and Chrysanthemum 6 I use two types of milling media in my 3" ID jars. One type is 3/4 " brass bar stock cut into 3/4" lengths - small cylinders of brass. The second type is .5" caliber lead shot. In each case I charge the jar half full of milling media and a further quarter of the jar of composition.

I prefer the brass bar stock because I can put in charcoal just as it comes out of the retort - no pre-grinding required - the cylinder shape of the brass, it seems, grinds up the charcoal for me and saves me one step. As well, the black powder doesn't cake onto the sides of the jar when I use the brass bar stock. Because the media is large it's also a bit easier to handle when separating out the media.

The one down side to the bar stock is that I have to mill longer. Starting with coarsely ground charcoal I'll have hot powder in about 4 hours with the lead media. It takes more like 12 hours with the brass cylinder media. This is not due to grinding up the charcoal - that happens fairly quickly - less then an hour.

What I am wondering is what is causing the difference in the milling times. There are three variables - the brass media is in cylinders while the lead is in the form of spheres, the brass is quite a bit larger then the lead and the brass is somewhat less dense than lead. At this point I'm willing to discount the differences between densities since brass is about 75% as dense as lead and the difference in milling times is much more then that.

Is it the size of the brass media or the shape which contributes to the longer milling time?

If it's the size I can probably hang out at my favourite scrap yard until I find a smaller diameter brass rod. If it is the shape then I guess I'm out of luck because I like how the cylinders cut up the charcoal - it takes longer to have the cylinders grind up the charcoal but it's a lot easier for me.

Thanks in advance.
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