Consistency of the burn of the powder. With a cronograph it is seen as standard deviation and extreme spread, although SD has many other parts. Some powders are easy to ignite, so they burn better. Some are harder to ignite and won't burn completely. For example, my 222 Rem, I use blc2. It's a ball powder with a heavy deturant coating. Until it's compressed a little, my extreme spread is about 50 fps. When I reach the upper loading per the reloading book, my SD and ES go down and accuracy goes up. I have to use a magnum primer to make sure it ignites thoroughly. In my 45 acp, I use bullseye, a fast burning powder that is easy to ignite. Low SD and ES. You want all the powder to burn all of the time. If not, the bullet will be out of the group.