Its a big NO NO! The 5.56 has more pressure being a military round where as the 223 is a cavillian round. Now if the lower says 223 BUT the barrel is stamped 5.56 your good.
Like shooting my full size 1911's & glocks. The compacks just dont fit my hands well. right now the funnest to shoot is my GSG 1911 in .22. Its a blast to shoot.