Inspiration:
For this year's Three D Guns challenge, I am designing a shot gun inspired by the Fallout 4 game.
Modelling Process:
I commenced my modeling in MODO. Here I built all the parts to make the double barrel.

The design is far too simplistic with the silhouette needing more work as well. I don't get a post apocalyptic vibe off of this piece, almost seems more "steampunk" to me but even then a Steampunk shotgun would just be one from WWI with some filigree on it. Here's a homemade shotgun, which doesn't seem too difficult to make, especially in a world where access to good machining tools is difficult to come by.
https://www.thefirearmblog.com/blog/2017/03/31/building-simple-break-barrel-shotgun-scratch/
Considering how much time is left in the contest, I think you should look to refining your concept and starting over on this piece.