Originally Posted by robocop10mm
Top one appears to be an M-16 carrier. The different amount of metal at the rear is to trip the auto sear. Adds a small amount of weight that is desireable
Nope; if anything, it is part way between M16 and SP1. An M16 will have the cutout on the bottom flush with the cutout on top. As I said earlier, there are bolt carriers out there with the metal cut out half way between M16 and SP1. This is done to keep the carrier from either tripping the auto sear or a lightning link/DIAS setup. I think FTF industries is selling some of them as full auto carriers, while they really aren't.