Breech Box for SIG 510 / Swiss Army Stgw57 (Sturmgewehr 1957). 

I printed it with one peripheral, two bottom and and two top layers and 15% infill, and it seems to be stable enough this way.

As always I am including the OpenSCAD source file for you to adapt and/or improve:

• Install OpenSCAD from https://www.openscad.org
• Launch OpenSCAD and open the enclosed OpenSCAD source file (/File/Open).
• Amend the parameters at the top of the file or even amend the code.
• Save your changes (/File/Save).
• Render the design (/Design/Render).
• Export the rendered design e.g. as a .stl file (/File/Export/Export as STL).
• Slice and print.

See my other designs at
https://www.thingiverse.com/thinger13/designs