It's an Hamita's manga, so you should read it as if you were reading something from G.R.R. Martin but without surprise resurrections and with way more cruelty and sad stuff at the end.
Maybe it's a reference to the 1997 french movie Dobermann here is it's wikipedia entry
It's a brutal action/gangster movie where the main character (nicknamed Dobermann) use a heavily modified gun, with some stylish gun-kata style fights.
Many french movies had been very popular in Japan, so...
Maybe it has been published in Light Novel version (physical book release, with sometimes heavy editing and re-writing), some authors don't have time to continue to put up new webnovel chapters with all the rewriting of the LN and the different continuity between the 2 versions of the story.
It was a nice one too bad it was too short and,unfortunately, the novel version ended around the same as the manga version, so no novel continuation here...
That's so sad...
Yeah, once again it's the 3 days of the year when you can't read any new chapters (because april fool last 3 days on the World Wide Web, and all teams work hard on their "joke" releases instead of actual ones...)