CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
PolarBear,
Yes. Good decision.
Nobody can tell you what a +P+ pressure is because it doesn't have an assigned one. The designation is broadly used by manufacturers to indicate ammunition not loaded to SAAMI standards. That can mean higher pressure, but it doesn't have to. SAAMI, for example, has velocity standards for each bullet weight. Someone on another board with a Pressure Trace measured a manufacturer's +P and their +P+ loads in a carbine and found the +P+ actually produced lower peak pressure than the +P load, but it's velocity was higher and outside the +P SAAMI velocity range for the bullet weight. It was probably a load worked up with slower-than-normal-for-38 Special powder and a magnum primer and an extra hard crimp to get the powder burning.
Otherwise, if you wanted to make a guess at what manufacturers might use by way of a pressure limit, I think you can look at the .38 Special proof loads. They won't want to get to that range as a steady diet of proof loads can wear out a gun, stretch a frame or cause other grief if the gun isn't built heavily enough or with hard enough steel. The original proof loads for .38 Special are now obsolete and +P proof levels are currently used for both .38 Special and .38 Special +P. The minimum proof pressure for .38 Special and .38 Special +P is 27,000 psi. I think anybody manufacturing .38 Special who exceeded that pressure number would be asking for legal trouble. If I were in charge of an ammo plant, I wouldn't let the +P+ loads exceed +P by any more than +P exceeds the standard maximum pressure, and that difference is 3,000 psi. In other words, 23,000 psi would be my in-house limit. It also just happens to be the lower proof limit for the now-obsolete .38 Special standard. But I don't run an ammo plant, so I can't say for sure what some manufacturers may actually do in some instances.