I've forgotten the weights now, as I haven't cut these molds for some time, but length is the same as diameter with all calibers. ( I make them for 30 caliber up to 50 caliber, for rifle and revolver use.)
I'm going to guess weight will be not less than 10% more, or more than 20% more than a round ball.
The flat, I can't eliminate with my cutting method which is lathe boring, but it can be down to about 1/8 inch which is hardly noticeable, and of no effect on performance, or it can be as large as the customer wants. The benefits of a large flat, or meplat, is it allows more powder space, because the BB is seated out farther, whereas a BB reduces powder space a bit compared to a round ball. Of coarse the second and greatest benefit of a good sized flat is killing impact.
I mixed the powder together dry. Just shook them up a little in the powder horn. This for ease of loading. Once mixed it is nearly invisible in the black and would probably require a magnifying glass to be seen for most people, so be sure it doesn't get mixed up with all black powder. For the max loads with mostly Unique I used just enough black to get ignition, the rest Unique. Velocity was 1100 fps in from the Ruger, which is far from a high pressure load, yet too much for many weak black powder revolvers. I'm guessing pressure was well below most standard velocity 38 special loads, and the report was not loud at all. Just slightly more than with FFF.