The M-16 is called a recoilless rifle for a reason. It was designed to have hardly any recoil. It hasn't changed between now and then and certainly not in 1986-1987 when King was writing Drawing of the Three. Also the recoil of any gun does NOT depend on the experience with the gun. How you handle the recoil does. I've shot a lot of guns and several different AR-15s various years apart in manufacture and their recoil doesn't change.
King has never known guns or made any attempt to. Him making some guy firing an M-16 out like he was firing an Thompson gun which from all accounts DOES have a big recoil especially the 45 cal ones, was just another example of it.