For the longest time turrets in fallout have been ... well to say it bluntly small and under-powered, you imagine something bigger to defend a settlement and with all the old world military tech being left over... its not beyond thinking some of it might have been put to use. 

These resources give a Gauss Cannon, A base with motor assets from the original fallout and a heavy steel frame for holding and housing such a weapon.
Theirs not much else to say about it really. 

I don't have the skills or as of late the time to Finnish this off and i'd love to see it in action.
I give permission for anyone to use and adjust this, just please credit me along side you if you Finnish this off. 

The lore in my mind for something like this is ... 

If you side with railroad/min men then along your time visiting settlements you take Sturges along while patroling and while on the way to pay a visit to the Mechanist (Isabel Cruz), you stop by North Chruch and have a word with tinker tom who has an idea for a more heavy hitting weapon but the tools he has at HQ limit him, knowing of Mechanist facility from your talks, he asks to go there with you. Long story short after visiting it with the two, keeping quiet about who tom is... they soon get to work on creating this weapon with the help of the advanced facility. 

I had orignally planned to make a whole quest mod around this but sadly ... im a noob so I can't. 

I don't know what else to put here so I'll just thank you if you take on this project.
Also I'll put this in here, i'm a 3d artist and if you're in need of a asset making, provide the refrence images and i'll do my best, I need more pratice with game assets, textures too.