What is the meaning of Rivet_gun?

A tool used by a riveter for joining pieces of metal together with a rivet.

1. An adjustable-impulse pneumatic hammer, fitted with a rivet set which matches the shape of the factory head of a solid rivet, used by a riveter (or gunner) to impact the factory head of that rivet (which has been heated if necessary to soften it), while a heavy bucking bar or similar is held against the bucktail of the rivet, upsetting it to form the field head.
2. A ratchetting lever mechanism which is attached to the frangible mandrel passing through a typical blind rivet, which when pulled upsets the bucktail to form the field head of the rivet without needing access to the field-head side of the construction.

An adjustable-impulse pneumatic hammer, fitted with a rivet set which matches the shape of the factory head of a solid rivet, used by a riveter (or gunner) to impact the factory head of that rivet (which has been heated if necessary to soften it), while a heavy bucking bar or similar is held against the bucktail of the rivet, upsetting it to form the field head.

A ratchetting lever mechanism which is attached to the frangible mandrel passing through a typical blind rivet, which when pulled upsets the bucktail to form the field head of the rivet without needing access to the field-head side of the construction.

Source: wiktionary.org