The burn rate of a powder is the rate at which the powder itself burns and the characteristic way in which it generates a pressure curve from chamber to muzzle.
The velocity is the velocity of the bullet at the muzzle. So burn rate is the powder, velocity is the bullet the way I am using it.
The velocity produced by a powder on a given bullet is gonna depend on the weight and friction of the bullet, but also on the pressure curve a particular powder produces in the rifle.
Chemicals added to some powders may end up retarding pressure curve and bullet velocity even if the burn rate of that powder is faster than another powder.
Shape of the powder, ball or stick, and density packed in the case also has an effect.
Hope that helps,?
YES....if you experiment with reloading you better know !