[Unclenick]

A faster powder burns more completely before the bullet has moved as far forward as it will when the same percentage of a slow powder charge has burned. This means its gas is evolved in a smaller volume, so that if it made an equal quantity of gas to that made by a slower powder, the fast powder's peak pressure will be higher. As a result, you can pack more of a slow powder in a case, and if you pack enough in to reach the same peak pressure as you got with a fast powder load, the slower powder will get you a higher velocity. That is because its larger charge has made more total gas which has maintained driving pressure longer. It's pressure peak just won't be reached as early in the bullet travel, so it peaks in a larger volume than the fast powder does. That extra gas needs that extra volume to avoid exceeding pressure limits.

When both a fast and a slow powder produce the same velocity, the slow powder will be doing so with a lower peak pressure. The actual burning rate achieved in your gun (not the order on the burning rate charts, necessarily) will depend on pressure. Chemical reactions generally speed up with pressure. Thus, the slower the powder, the more it needs pressure burn completely. So, when you use a lower pressure with a slow powder to obtain the same velocity as you get at a higher pressure with a fast powder, you get less complete burning, tossing more unburned powder out of the muzzle with the bullet. You also get more fouling, because of the less well-combusted chemistry, and often also get more erratic muzzle velocities because small, cartridge-to-cartridge physical differences have a disproportionate effect on getting the powder burn established when the pressures are marginal for good burning.

Generally, the only reason to use a slow powder is to take advantage of the higher velocity it can produce at full pressure, so it is only showing its value at the warmer end of the cartridge's pressure rating.