I've been making BP for a couple years now (small amounts, a couple pounds at a time) for non-cannon use. There can be a significant amount of variability in powder performance due to availabilities in raw materials and process.
The type of charcoal used can make the performance vary by roughly 2/1. An experienced manufacturer should be able to figure out what works best and reduce that variability by quite a bit.
The process (generally milling, then corning, then granulation and coating) can probably affect things by 5/1, but I would think if you are a powder manufacturer you have that mostly under control and it is maybe closer to 2/1. Typically, the longer you mill it (up to a point), the better it works. However, the longer you mill it, the more it costs to make. Even though milling is pretty much a hands off process except for loading and unloading time, machine time costs money too. They tend to separate mills by some distance for safety reasons, so adding mills not only involves buying a mill, but buying a place to put it.
So, even with experience there is room for variation based on your raw material source and the manufacturing capability variation and manufacturing cost factors.
This is at least a partial explanation as to why the powders that seem to be more powerful and more consistent tend to cost more.
Rick
