It does take some time to set up for carrying out this type of testing and collecting the data The powder was weighed out for each charge but the balls were only taken out of a random selection of my own cast balls. The calculation was taken on the average weight of the balls.
If you wanted a more accurate set of data you would ensure all the balls were of the same weight and the pressure used to push the balls down was also controlled by loading the cylinder out of the frame in a jig.
As we only required this to illustrate the effect the difference in the powder charges and the types of powder make, it is close enough for that purpose. As you can appreciate there is always a slight variation as to how much pressure you would use in pushing down the ball on the powder charge. Also note the finer the black powder is, the faster velocity is recorded. This is because the finer the powder is, the faster it burns and the faster it burns, the quicker it builds up pressure and increases the velocity.
When shooting indoors many shooter reduce their powder charges to as little as 12 grains or less. It is important to be very careful when doing this, as if you under load your pistol too much, it can result in detonation and the barrel might burst or explode. To little velocity can result in the ball rebounding from the target area and coming back towards the firing point.