A link to many of their sites can be found by clicking here. Black powder is a mixture of sulfur, charcoal and potassium or sodium nitrate. A gun powder could be black powder, but it can also refer to many other chemical combinations. Black powder is sold and specified using two parameters, its weight and the size of the granules. Black powder weight is measured in different units.
Modern shells have the weight listed in Drams.
We sell a very nice, adjustable black powder measurer in our arsenal store. One is certainly worth having. The granule size is referred to as the fg number. It correlates to the size of the screen mesh which it falls through for sorting. The resulting explosion from very fine powder can cause a cannon barrel to explode.