OK, how can you have 40% more pressure, but no increase in velocity? Something to do with the pressure curve? Maybe a huge spike before the projectile started to move, which spent itself out the vent or past the projectile.
Good question 
Easy. THe pressure spike occurs for a VERY short period of time. Enough to test the metal for sure. But the ball moves and accelerates for a MUCH longer period of time. Hence the need to use a coarse powder that will maintain the volume pushing the ball out the tube.
Interesting...
I'd think that determining optimal powder grain size/burn rate for a particular barrel and projectile would involve:
1. Bore diameter
2. Bore length
3. Projectile weight
4. Windage (Plenty with a smooth bore, none with a rifled bbl)
5. Reduced powder chamber diameter/no chamber
6. Resistance to the projectile moving through the bore (Which could be significant in a rifled bbl using a projectile with a lot of area contacting the bore)
If I'm not mistaken, the volume of gas produced by a given weight of black powder is equal regardless of grain size. The speed at which the charge burns determines how quickly that volume of gas is produced. That's why you can build a model rocket motor with one "grain" of black powder and not have it blow up on the launch pad; it burns relatively slowly.
In a long smooth bore bbl using a heavy projectile with windage, I can see the benefit of a coarse powder/slower burn so that the gas isn't produced too rapidly (creating higher initial pressure) and blown past the projectile and out the vent before it starts moving significantly. Using a fast-burning powder here wouldn't be efficient as it would produce more pressure but may not increase velocity significantly, as some of the expanding gas is wasted.
On the other hand, using a slow burning powder with a light projectile from a short bbl could also be a waste, as much of the powder might be unburned by the time the ball is long gone.
