When a firearm has been so tested, the proof house marks it by stamping one or more symbols onto the gun.
This is often done in government-operated 'proof houses'. Special extra-hot 'proof loads' are fired through all guns to indicate their ability to withstand regular pressure loads over long periods of time.
Industry: governments often set standards on the amount of pressure gun barrels are required to withstand before bursting. Quote: SIG-Sauer Proofmarkings European firearms manufacturing tends to be more heavily regulated than the U.S.