New_Texan

Ok, so I'm a newbie to shotguns (guns in general). So, please when answering my question, layman's terms! I picked up my M4 today at a gun show. It is new, never been shot, but was assembled for the show. I read the manual. Started to field strip it (to get familiar with it). All went fine until the time I tried to re-assemble it! I believe that bolt assembly is seated properly, but for the life of me, I cannot seem to get the barrel to "snap" into place. What am I doing wrong?!