It relates specifically to pressure. There is an American industry standard pressure; the average maximum working pressure for each cartridge that is produced. A "+P" or "+P+" are steps up in the maximum average working pressure. Increased velocity does generally result - but not absolutely. Some +P loads sometimes, although rarely, do not produce more velocity than the standard loads of other manufacturers. This is probably in cases where the standard load uses a better powder, where the increased efficiency and particular charactaristics increases velocity without a significant average pressure increase.
The europeans have a similar standard. They use metric pressure measurements, and their standard pressure loads may differ alittle from those made in the USA. Norma and Fiocchi are two that come to mind. Though I am not 100% sure that some of their "hotter" loads produce more velocity because of better powder as opposed to simply loading them to higher pressure standards.
Of course they have to take into account that even the same make, model, and batch of any given firearm have very minor dimensional differences in chamber and bore within a tolerence, not to mention the variances in the components of the cartridges themselves. So chamber pressures may very slightly from gun to gun, especially between different manufacturers.
Talking of industry standards for cartridges; isn't it interesting that the industry (as in private) has done this for a very long time. It has done the same for the firearms that use them - all without any government involvement or "regulation". At least up to now where States like California have introduced those assinine and meaningless test standards to "save us" all from cheap affordable guns. I suppose the California legislature was concerned about the idea of poor people being able to defend themselves.
But then certain elements in our government(s) do have a subtle way of subverting civil liberties without actually coming right out and saying that is what it is doing.
Oh ... how I LOVE big brother!
[This message has been edited by LAK (edited 09-22-2001).]