RockymtnTactical - that generalization could be dangerous. In the sense of handloading, more pressure doesn't always equal more velocity. Powder burn rate affects pressure and speed. A fast burning powder will reach peak pressure faster, but it will not sustain that pressure for very long. This means fast acceleration at the beginning, but letting off the gas so to speak before the bullet leaves the barrel. Slow burning powders reach peak pressure later, but maintain a higher pressure longer. This means the bullet accelerates a little slower (relative here) but for a much longer time period, which results in faster exit velocities and lower pressures.
For example, my fastest MV load data for my .308 actually is 5-10k lower peak pressure than some of the slower MV loads that I have, depending on the powder used.
AUN - +P is a marketing gimmick for the most part. SAAMI has max pressure for each round. Most manufacturers don't load to the max pressures. The +P loaded stuff is closer to max SAAMI pressure (when you are purchasing it from stores). There is an actual spec for .38 spc +P as well as a 9mm +P. When handloading, it is feasible to exceed the .38sp pressures and still be within the .38sp +P max pressure ratings. IF I had my load data here, I could probably give you a load that meets the "+P" speeds that companies are listing without exceeding the regular pressure limits. There are exceptions, but for the most part the +P stuff is closer to the hot end of the regular specs. instead of surpassing it when it is factory loaded ammo.
BTW - SAAMI max pressures difference between .38sp and .38sp+P is 1.5k psi, which isn't much. .45 and .45+P is 2k psi. That, in general is about .1 grains difference in powder charge or less.
