The honey-combing of gun-barrels is caused by the residue, left by the exploded powder, setting up a galvanic action between the iron and steel composing Damascus and laminated steel barrels, or between the different grades of iron forming twist barrels.
Source: wiktionary
A petition from the master gun-barrel makers of Birmingham and district to his Majesty's Secretary of State for War praying that steps be taken to amend the present practice of proof-marking foreign-made barrels with the same proof-mark or marks as those stamped upon barrels made in the United Kingdom, and to adopt a mark or marks which would be indicative of foreign make, and which would be easily discernable by sportsmen and other purchasers and colonial and foreign Customs officers.
Source: wiktionary
If, on the other hand, we perceive the smoking gun-barrel as proof of something then we are precisely trying to rule out alternatives.
Source: wiktionary