Some of the lowers are out of spec preventing assembly. You might want to check them if you were putting it up for a future build or before you get to far into one. Mine looks alright mated but I'm waiting on a lpk and will try it with just pins no detents next week. http://www.ar15.com/forums/t_3_4/594209_AIM_Surplus_AR_FIFTEEN___aka_AR_57__out_of_spec_issues_.html http://www.ar15.com/forums/t_2_217/227990_AR57_Lower___Out_of_Spec.html Wanna kill these ads? We can help!