Who,
As a general rule, lighter bullets work better with faster burning powders, and heavier bullets work better with slower burning powders. +P designates peak pressure. Faster buring powders tend to "spike" earlier and have a shorter power/pressure curve. This makes them useful in shorter barrels since it allows more powder to be burned, and thusly more velocity potential. The peak pressure can sometimes be reached before the bullet has entered the forcing cone, and will fall off as the bullet travels down the barrel.
With a slower burning powder, the power/pressure curve is spread out over a longer time period, and they tend to deliver more velocity out of a longer barrel, where more of the powder can be burned. Peak pressures can occur sometimes as late as near the muzzle,,,,often, as with the Fiocchi, peak pressure can occur somewhere at a point where the bullet has left the muzzle. The large flash indicates unburned powder. If you had more barrel length, there would be more powder burned and thusly higher velocity.
The best analogy I can give is a comparison between a squirt gun and a fire hose. For the sake of discussion, let's assume you could buy a squirt gun that operated at the same pressure (psi) as a fire hose. If you squirted someone, all they'd get would be a small wet spot on their shirt. Now hit them with a shot from a fire hose,,,at the same peak pressure level,,,and it throws them backwards. The reason why is obvious. The fire hose has a longer duration of pressure, as well as more volume.
Now if we apply that to a 125 gr w/"fast" powder and a 158 gr w/"slow" powder, the 125 gr will peak in a shorter confined area, thus have a higher pounds per square inch figure. The slower powder will peak in a longer area, and have a lower pounds per square inch figure. Same (or close to same) pressure, but higher volume of gas, and longer duration of applied pressure.
(For the record, I agree with anyone that says my above explanation has more holes than a Clinton alibi, and it's a gross oversimplification. Hey, there's ton's of books and reloading manuals that deal with it, and I have what,,,,100 words or so to work with here?)
