The chamber is clean and the entire rifle was clean, its got me stumped, maybe the headspace is off?? im not sure, sometimes it has trouble seating a round in the chamber, maybe the chamber is reacting to the heat of the first 5 rounds?
I seriously doubt its a heat issue...especially after only five rounds. You need to take apart the gas system and re-clean it. You never said if you were able to clear to gas port with a pipe cleaner. There may be serious copper fouling or some other piece of crud obstructing the gas port from the bore side of the barrel.