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During my 300 mile drive home a couple days ago I was thinking about why certain gunpowders are used for certain situations and I think I have finally figured it out.  Here is how I currently understand it to work, please correct me if I am wrong.

As a gun is fired and the bullet makes it way down the barrel the volume of area behind that bullet increases until it exits the muzzle. The highest velocity will be achieved with a particular cartridge/bullet/barrel length combination when the type and amount of powder being fired produces gases at the same rate as the area behind the bullet increases.  For example the max pressure for .308 Win. is 62,000 PSI.  We should get the highest velocity with the amount and type of powder that can maintain 62,000 PSI for the entire time the bullet is in the barrel (or as close to that as we can get).  Using a faster or slower powder, or using more or less of the same powder will only result in either an over-pressure situation or decreased velocity.  

For example if we use the same amount of a faster powder, gases will be produced at a rate faster than the volume behind the bullet increases, which is cause too much pressure.  If we use the same faster powder but use less of it to stay under max pressure the powder will be all burnt up and pressure will drop before the bullet leaves the muzzle, resulting in lower velocity.  If we use the same amount as the optimal load of a slower powder it won't produce gases fast enough and the pressure will never reach max, thus slower velocity.  And lastly if we to use an larger amount of the slower powder we will probably be out of case capacity and won't be able to cram it all in the case.

If the above statement is what happens in reality we can draw several conclusions that I think are very useful and are commonly misunderstood.

1. If a load provides optimal performance in a longer barrel, the same load will provide optimal performance in a shorter barrel.  You won't be able to get any more velocity by switching to a faster powder in a shorter barrel.  The reason for this is our optimal load is pushing at max pressure (or as close as we can get to it) all the way down the barrel, with a shorter barrel this will still be the case so there is no room for improvement.  What you may be able to do though is achieve the same velocity with a smaller amount of a faster powder and thus be a little more efficient.  This is because with a shorter barrel the duration that the powder has to push against the bullet is shorter so we can use a powder type and quantity that pushes just as hard but for a shorter amount of time. A smaller amount of a faster powder should do this.

2.  A load that provides optimal performance in a shorter barrel may not be optimal in a longer barrel unless it is already using up all available case capacity.  For example if we have a load that gives us optimal performance in our hypothetical 4" .44 Mag and only uses up about half the case capacity that load probably will not be optimal in a 8" barrel because the powder will burn up and pressure will drop before the bullet leaves the muzzle.  In this case we could switch to a larger amount of a slower powder and get better performance.

3. Heavy for caliber bullets will suffer less of a loss of performance when going to a shorter barrel than a light for caliber bullet.  This is due to the smaller case capacity and increased time in the bore when using heavier bullets.

4. A wildcat cartridge with a larger bullet diameter but similar case capacity to the parent cartridge will benefit from a slightly faster powder when using the same bullet weight as the parent cartridge. This is due to the increased volume behind the bullet as it travels down the bore requiring a higher gas volume.

5. A cartridge that reaches its max performance and still has case capacity left over is probably overbore and would benefit from a longer barrel, thus allowing you to use a larger amount of a slower powder.

6. Once an optimal load had been found for a particular firearm the only way to increase velocity is to increase pressure or increase the time the bullet spends in the barrel, either through using a longer barrel or a heavier bullet.  Increased powder capacity alone will not help, it will only make it (more) overbore.

Hopefully my giant wall of text didn't frighten too many of you off!  Feel free to tell me if and how I am wrong about any of this.