American factories prove firearms in the interest of safety. Foreign governments do so both for safety and to control the manufacture of firearms and, sometimes, prevent the importation of arms that would compete with local manufacturers. (The main purpose of the original British proof law was to eliminate Belgian makers from the English market.)
No one uses proof loads of 50 or 100 percent over maximum average. U.S. proof loads run 25-30% above, just as European loads do. The idea is to weed out defective guns, mainly gun barrels, not to blow up half the production. In reality, a gun failing proof is a very rare occurrence today with modern steels and manufacturing methods, but in the days of Damascus barrels and iron frames and cylinders, proof testing was needed, in good part just to reassure the customer that the gun was not dangerous.
Aside from proof testing, which is done for every gun, manufacturers periodically run destructive tests, deliberately trying to find out how much pressure is needed to actually blow up a gun and where improvements can be made. Remington has reportedly tested the Model 700 to 175,000 psi without a failure, but that rifle has a very strong action. Other rifles should still handle much greater than SAAMI average maximum without failure.
The weak point in any rifle is the cartridge case. At very high pressures, brass acts like playdough, and a blown case head will almost always wreck the rifle.
Jim