i just got in trade an ar15 with an unknown upper on a bushmaster lower. it has no markings what ever on the upper except for the barrel.it is marked with ss and the caliber. the barrel is black and a magnet sticks to it.the front hole for the pivot pin is larger than the hole in the lower and fits rather loose. it is not as big as .312 but is larger than .250
can anyone help me decide what i have?