Here is a neat little design for a very simple homemade M11 clone sent in. It is assembled mostly from pieces of steel box tube and bar stock telescoped together.

Bolt assembly

Trigger grouping

The prototype shown was built as a non-firing display model with a dummy bolt and barrel for legal reasons, though it perfectly demonstrates how a compact commercial design can be constructed with little more than a drill, angle grinder and welder.

PDF:  The Box Tube MAC-11 – Practical Scrap Metal Small Arms Vol.II

…. and part 2

Note:  These schematics are for academic / technical study purposes only.