I burned FG black powder (90 grains) in clear plastic tubes, ½ inch in diameter and 1 1/8 inches long. I video the burns at 1200 frames per second and initiated the burns with cannon fuse. The fuse was placed at the bottom, top and middle of the powder column. A new tube was used for each replicate and fuse position. The following are the average (5 replicates) mass burn rates under the conditions described:
When the fuse was located at the bottom of the tube the powder burnt at 1240 grains per second. Shortly after ignition the powder column was pushed from the tube and burn very rapidly.
When the fuse was located at the middle of the tube the powder burnt at 610 grains per second. The powder column above the fuse was ejected from the tube shortly after ignition and burned very rapidly. The powder below the fuse did not start burning until the ejection of the powder above the fuse and then burn down the tube.
When the fuse was located at the top of the tube the powder burnt at 281 grains per second. The powder burnt down the tube with no ejection of powder particles.
While these results can not be directly related to velocity they indicate black powder at least in a chamber environment and at atmospheric pressure can burn at different rates depending on the ignition point.