FWIW, there are only a few "real" +P loads- 9mm, .38 Special, .45 ACP, and ALL .38 Supers. Those are the rounds for which SAAMI established a +P load. They are loaded to about 17% higher pressures IIRC. While someone may CALL another cartridge a +P, there is no standard for other cartridges.
The +P+ is loaded hotter than a +P, but again, there is no standard for that load- so in theory, you would load that up as a 2 times pressure load.
SOME handguns- especially older ones, were not designed for the higher pressures. Unlikely to have an OMIGOD! kind of failure, but will cause accelerated wear on the gun.