Interesting comments, as the "P" on U.S. rifle stocks really does mean the rifle was proved with an overload, just the same as in any other proof test. The reason the proof mark was put on the stock was that U.S. actions were very hard (or case hardened) and a proof mark would not take well.
In addition, barrels were proved not once, but at least twice. The billet was drilled, short chambered, then installed in a test jig and a proof load fired. That was to weed out defective billets before time was wasted in finish boring, reaming, rifling, turning, and chambering. Then, after the barrel was finished, it was again proof fired in a test jig. As before, the idea was to weed out any bad barrels before time and money was wasted in installation of a barrel and also because if a bad barrel was installed and failed proof, it would normally take the receiver and stock along when it blew, with even greater waste.
The barrel proof marks varied; one was to put a dot in the middle of the ordnance "flaming bomb"; another was the simple letter "P".
Receivers were also proved with test barrels, again to weed out failures before time and money was wasted on them. The proof was usually a single punch mark, sometimes believed to be from a hardness test.
Like rifles, pistol barrels were also proof fired in a jig; the "P" on M1911/A1 barrels is a barrel proof, not a gun proof.