Cheap brittle plastic - AIM Sports Inc AR Upper/Lower Receiver Vice Block Set

I ended up returning this item and got a solid polyurethane upper vice block and an aluminum pin-type lower vice block on Ebay for a little more. The polyurethane is much stronger yet it's surface is softer than this cheap polycarbonate plastic.

I'm not sure which is better: by securing the upper by the takedown pins or by clamping the assembly from the outside, but the pin type is working just fine for me. There is no movement with it whereas the clamping one had a little play.