Proof Load
Also: Proof Round
A deliberately overpressure cartridge fired to test the strength of an action and barrel, after which the firearm receives a proof mark.
A proof load is a special cartridge loaded to a chamber pressure well above the normal service maximum, typically on the order of 25 to 30 percent higher. It is fired once in a new firearm under controlled conditions to confirm that the action, bolt, and barrel can safely contain a worst-case overload. A firearm that survives the test without damage or measurable distortion is then stamped with a proof mark certifying it.
The procedure also verifies that the dimensions stay within tolerance after the stress, which is why proof firing is followed by checks of headspace and bore condition. In C.I.P. countries proof testing is a legal requirement carried out at an official proof house, while in the United States manufacturers proof their own firearms voluntarily under industry practice. Proof loads are never intended for ordinary shooting and are not sold to the public for that purpose.