The M16/AR15 upper and lowers in milspec form are made from 7075T6 (might be T651 or some other specialized heat treatment, I am not sure off hand). Others have made them from 6061 Al.
The chamber is part of the barrel, which is made of some form of steel. Thus, the upper receiever doesn't contain the chamber pressures. What it does do is control the bolt carrier assembly when it recoils, which is pretty darned vigorous - I wouldnt trust my face to a plastic upper.
Also, note that plastic firearms have been around for YEARS. My dad's first rifle, in the later 50s, was a Remington Nylon '66, which was except for a steel stamping for a cover, a steel bolt, and the steel barrel, made of Zytel.
The chamber is part of the barrel, which is made of some form of steel. Thus, the upper receiever doesn't contain the chamber pressures. What it does do is control the bolt carrier assembly when it recoils, which is pretty darned vigorous - I wouldnt trust my face to a plastic upper.
Also, note that plastic firearms have been around for YEARS. My dad's first rifle, in the later 50s, was a Remington Nylon '66, which was except for a steel stamping for a cover, a steel bolt, and the steel barrel, made of Zytel.