To add to MickeyD's absolutely correct explanation, muzzle blast is not related to powder type. It is a function of remaining pressure at the muzzle. Peak pressure is achieved generally within a few inches of the chamber - although burn rate does affect this, it always happens quite soon after the bullet leaves the case mouth.
After that moment of peak pressure, the pressure decreases as the swept volume of the bore increases. An analogy being the volume change as a piston moves down a bore. As long as the pressure is high enough, the bullet will keep accelerating. However, in a long enough barrel, pressure would drop to a level that the bullet would begin to lose velocity and eventually come to a stop due to friction.
Muzzle blast, however, is mostly a function of barrel length. The shorter the barrel, the higher the remaining pressure when the bullet clears the muzzle, and thus the louder the blast. Less time and distance to accelerate is also why velocities are lower from a shorter barrel. But as this is about blast, the bottom line is that shorter means louder.
Correct.
No free lunch.
