- Regelbau 611 -
The Regelbau program was a project by Germany in the second world war. It was a series of standardized bunkers designed to be built in large numbers in the Siegfried line and the Atlantic Wall. There were multiple strength levels at which these bunkers could be built, with the 611 being level B (2.0 meter thick walls) and being primarily designed for the army(Heere). The 611 was a "Flankierend Schartenständ", or flanking gun bunker. It also had crew quarters, with enough room for 6 men.

- Schematic -
The schematic itself is made in Minecraft 1.16.5, and is shown in the superflat world. The images in the more detailed map (map) are made in 1.7.10, but this version doesn't have "clean" stone stairs and slabs. You can solve this problem with some mods like Staircraft and Slabcraft. The gun itself is from the McHeli content pack:
McHeli mod
McHeli German WW2 content pack

If you have any tips don't hesitate to tell me in the comments!