(from ebookjapan):

The "first" spin-off of Golgo 13, long-awaited and exciting all over the world!　

The first spin-off serialization of the nationally popular Gekiga "Golgo 13" has finally been published in tankobon!　

The main character is a man who has been making Golgo 13's superb sniping possible for many years
earning large rewards and great trust from Golgo 13... Dave the gunsmith!
"There are some things that a gunsmith cannot compromise on."

Here is a hard-boiled drama that approaches the main story of "Golgo 13" from a different perspective!