I've got a New Frontier Armory polymer lower I got a few years back. It has worked fine for me. It's been under 5.56, 300 Blackout, and .458 SOCOM uppers. No issues or problems with performance. My only complaint is that the opening on the raised boss for the bolt release is just slightly wider than it should be. The result is some minor back/forth slop with the bolt release. It's a minor issue considering the cost of the lower.