There's not a ton written about this that I can find, but cracks are appearing in the US military's plan to replace the M4 with a heavier battle rifle designed to defeat body armor. Sig Sauer, which also holds contracts for the current generation pistol and LMG, won a contract last year to also provide the US military with a new rifle. This rifle is to be called the XM7. The civilian equivalent is the Sig Spear. Calling this thing an assault rifle is a bit of a stretch, as it is very heavy and uses heavy ammunition. It turns out that the rifle is failing to penetrate modern body armor without the use of tungsten bullets. Also its reliability is questionable.
One thing that I still don't understand is why they went with a steel head on a brass case rather than just a full steel case. Steel and brass have similar density, so steel case of the same thickness would have roughly the same weight. Are they able to make brass cases thinner? Or maybe the superior recyclability of brass cases was a concern?
Brass is used in a lot of mechanisms where friction needs to be reduced. It is kind of like the Teflon of metals. As a relatively soft metal, if you have a mechanism where brass interfaces with a harder metal, the brass will incur the majority of the wear, without gumming up the works nearly as much as aluminum (another relatively soft metal).