Sheet metal, made by either HK themselves or one of their licensed factories, such as Hellenic Arms of Greece, FMP or Portugal, and a few others not in the USA.
A good alternative is the cast stainless steel receivers made, first, by Special Weapons LLC and now by Century Arms.
The Federal Arms cast aluminium receiver should place ahead of the Hesse stamped receiver. Take a look at a bare Hesse receiver for why.
