No idea of the quality here but these lowers are cheap. Order here: http://www.tnarmsco.com/ar-style-hybrid-lower-any-color/# and place FB2015 in promo code box to get the dirt cheap price. They appear to be nylon with brass thread inserts for strength but they have a lifetime warranty. I was told there is also free shipping on orders over $70 but can't confirm that on their website.
https://www.facebook.com/TennesseeA...4780339670728/504780059670756/?type=1&theater
https://www.facebook.com/TennesseeA...4780339670728/504780059670756/?type=1&theater