Stoner designed the AR-10 with a steel barrel.
The military built an experimental barrel of alumimum swaged around a steel liner with rifling to reduce weight. In an extended firing test, the barrel heated up, the aluminum expanded away from the liner, and the barrel ruptured.
There were also fiberglass magazines for the AR-10, as a compromise between the flimsy aluminum mags and heavier steel ones. The HK also has very light aluminum magazines with a waffle pattern, the "paratrooper magazine".
