So when it says 'Start Charge', that means that it is a light-recoil and low velocity charge?
Ughh, it would be lighter recoil and lower velocity than max charge load that was "tested" to provide high enough pressure for reliable ignition and consistent chamber pressure for accuracy. But you also need to note the type of bullet profile used and the OAL length indicated for the start charge because how deep the bullet is seated directly affect chamber pressure (Example - For the same bullet, longer OAL will give you lower chamber pressure and lower velocity).
And when it says 'Maximum Charge' and has those higher velocity, then it means +P? Or is does +P negate velocity and means only high pressured??
More charge will result in higher velocity. The Maximum Charge (also known as DO NOT EXCEED CHARGE) with the indicated OAL for that particular type of bullet profile is CRITICAL because exceeding this charge/OAL will result in too much chamber pressure. Instead of pushing the bullet down the barrel, the higher pressure may cause the chamber/barrel to explode.
As to +P rating, someone else could probably do a better job of explaining. My limited take is that +P rating started with older 38 Special round that had an established "factory" pressure for the given strength of revolver frame (this did not mean the "factory" pressure exceeded the case pressure max rating). As some stronger revolver frame was introduced, ammunition manufacturer could increase the powder charge because the stronger frames were able to withstand the higher pressure - they called these rounds +P (and later +P+ for loads for even stronger frames).
Unlike older revolvers that had large differing frame strength and separate cylinder/barrel, modern semi auto pistols have integrated chamber and barrel, thus able to withstand higher chamber pressure. Although there was an established "standard" factory pressure rating of "P" for earlier 9mm pistols, heavier designed submachine guns like MP5 are able to withstand higher than standard pressure for 9mm - these rounds received the +P designation and were not meant to be fired in a pistol.
When newer 9mm pistols like Glocks came to market, they were designed and built to withstand the higher +P 9mm ammunition meant for submachine guns. If you look at older pistol manuals, they indicated whether they were rated to +P 9mm or not. Nowdays, most manufacturers build their pistols to +P pressures and the designation became less important as most factory JHPs like Golden Saber and Ranger SXT (Black Talon) are in +P pressure/velocity. This is the reason why you see +P 9mm ammunition and some reloaders loading past the Max Load Data (but the published load data must be SAFE for all 9mm pistols old and new and indicates lower charges).
As to reloading past the Max Load Data pressures, it is the general rule among the reloading community that Max Never Exceed charges/OAL are never exceeded because many of us have seen the results of exploding guns with injuries to shooters. Because pressure buildup and spiking occurs more readily past the max charge loads, as a rule, I use 10%-5% (for 115gr 9mm FMJ reloads, I go up to 3%) less max load data (lead bullets are always loaded to 10%-5% less max).
I hope this helps. Perhaps someone could explain the +P issue better.
) The so-called "submachinegun" ammo brought back from WW2 was standard pressure European ammo with a harder thicker primer cup for use in fixed firing pin, open bolt submachineguns.
