Technically, there is no such thing as a .45-70 +P, That is to say, the SAAMI does not recognize such a thing and they are the only ones here who set the standards, which, by the way, are entirely voluntary. In other words, you can do anything you want and call it anything you want, and you won't hear from them.
According to them, the average copper units of pressure for a .45-70 is 28,000. A .458 magnum is 53,000 and there are some that are even higher. But the only rifle cartridge given a +P specification is the .257 Roberts. A .30-06 Springfield, as they call it, is at 50,000 but the .308 is 52,000.
Among handgun cartridges, there are +P listings for 9mm, .38 special and believe it or not, .45 automatic. The .38 Super is listed only as +P, which I had guessed at earlier. They do not recognize +P+ as a standard pressure.
For some reason I didn't see a listing for .450 Marlin. They also do not define magnum, either, and some very high pressure cartridges do not use that name.
According to them, the average copper units of pressure for a .45-70 is 28,000. A .458 magnum is 53,000 and there are some that are even higher. But the only rifle cartridge given a +P specification is the .257 Roberts. A .30-06 Springfield, as they call it, is at 50,000 but the .308 is 52,000.
Among handgun cartridges, there are +P listings for 9mm, .38 special and believe it or not, .45 automatic. The .38 Super is listed only as +P, which I had guessed at earlier. They do not recognize +P+ as a standard pressure.
For some reason I didn't see a listing for .450 Marlin. They also do not define magnum, either, and some very high pressure cartridges do not use that name.