I was taking a look at professor parabellum's box-tube Mac11 and I noticed everything is made of mild steel.

The SAAMI specifications specifies that 9mm creates 240 MPa(35000) force, while mild steel's yield strength(strength before the metal gets deformed) is 250 MPa(36000 psi).

I think that lower receiver and the magwell can be made out of mild steel without an issue since those aren't the parts that get stressed the most. However I would be too afraid to try this out for bolt, bolt carrier, upper or barrel. My solutions is to make a barrel out of 4140 pre-hardened steel, and the bolt out of the same material.

Could I leave the bolt carrier and the upper as mild steel? Or is it likely to blow up?