If I recall correctly, the test only involved other contemporary American doubles--all of which survived, at a minimum, dozens of proof loads. Given that the standard proof for an English game gun is about 5,000 psi lower than the American SAAMI standard, it wouldn't be quite kosher to subject them to the same proof loads as 21's. A gun that has passed CIP/British magnum/superior proof, on the other hand, is subjected to essentially equal proof pressures.