Fair upper - Radical Firearms 10.5 in. 5.56 M4 Upper Assembly

I purchased t bu is upper to complete my work gun. Everything works great and I do like it. However there is a problem with the barrel I encountered. While going through armored course I gauged the barrel and it is not 5.56 even though its stamped that way. Had few ppl check it since and they confirmed my findings. 5.56 gauge did not fit. Also I tried to put a 5.56 to .22lr conversion in it and that did not fit as well.