Barrel Proofing
Dad and I were browsing the used gun rack at a big box store on a trip through Pennsylvania a few years back when we happened upon a double gun that caught our eye. The gun was a 12ga with light color walnut stock and forend. On closer inspection, we saw a moderate amount of hand-cut engraving, as well as hand-cut checkering on the buttery walnut stock. An older gun made in Europe, the tag read 'BLNE', as it is sometimes referred to in gun parlance or a boxlock non-ejector. We speculated on the country of origins and details of the gun's specifications until we could get the attention of one of the clerks to ask if it was OK to take the barrels off the action and check the proof marks.
Spend any amount of time in a gun shop that deals with guns from Europe and you will inevitably hear someone say "check the proof marks". This statement broadly suggests checking the markings on the barrel flats and action watertable(on a shotgun). These are the two areas that gunmakers typically apply stampings pertaining to original specifications of the gun like chamber length or choke. It is also the area where proof houses apply proof marks on a gun.
Proofing is a type of (potentially) destructive testing whereby a firearm is discharged with appropriate dimension ammunition that has been overloaded with powder on purpose. Shooting a round overcharged with powder produces higher than normal pressure inside the barrels and action when the gun is fired. If the gun can withstand the increased pressure produced by an 'overloaded' round, it will withstand the significantly lower pressure of standard factory ammunition. Guns are measured before and after testing and fired remotely while being held in fixtures inside secured rooms for safety reasons. Proofing is a pass or fail test, there is no middle ground. Guns that fail may experience a bulged or split barrel, or in extreme situations, action failure can result in shattered parts.
Proofing firearms began hundreds of years ago in Europe(1637 in the UK) and continues to be conducted as described above. 14 countries in Europe have adopted standards laid out by CIP (think international proofing organization) which now dictate the pressures various firearms need to withstand to make proof.
Each proof house in Europe developed its own proof marks. These marks changed over the years and can help date a gun or determine a gun proofed with black powder or modern smokeless powder. Most European guns were proofed in the country in which they were made, or at least the country where they were assembled to the point that they could be shot. As a result, the proof establishes the maker's country, and in cases where countries had more than one proof house, will determine which proof house the gun was tested in. For example, 6 different German proof houses are Ulm, Hannover, Kiel, Munich, Cologne, and Berlin each of which has a different proof mark.