The ACI Modern Heroes Vol.2 Heads is made of plastic, just like other heads of other famous 1/6 scale brand. They can be used with all main popular 1/6 bodies like Hot Toys and DID where they are now the 2 main manufacturers of Modern, Movies, WWII, Napoleonic and Medieval Samurai figures. These 2 brands almost covered 70% of the 1/6 scale action figures markets and ACI Modern Heroes Vol.2 heads could be very good choices for collectors to customize their figures like US soldiers in WWII.  Compared to Vol.1, the neck joints of the heads are higher in compatability to the recent market trends which Hot Toys and DID are supplying most of the top quality products in the market! No additional tools and steps is needed when they are using with HOt Toys and DID  bodies, but only a hair dryer. For other brands, some minor steps or glues will be needed.
For their speciality, I found that the height of the neck is comparatively higher than many of the available heads in the market. This is really a very good improvement and merits that the figure using ACI Vol.2 heads will be a more "good looking" one especially when they are wearing thicker uniforms, etc. No More Short Necks! I suppose this will be the major trend of the coming figures because of a better "outlook" can be achieved. Don't you find that Hot Toys are using longer neck joints for the Dark Knight Batman! Or Russell & William from DID! Not all of us might observed that but is it "better"? So, might be these heads are really for the rising trend of nowadays action figure market.Many customers saying the real products are much better than their official pictures, I guess ACI needed to improve on that!
Hope it helps!