An update:
Now the news is telling us that three weapons were recovered: the shotgun he brought onto the base, and two handguns he picked up on the base. No automatic rifle.
Basically, a shotgun it would be legal to own even in England, and two harvested weapons, neither of them an AR-15.
If this is true (and given the media's performance so far, who knows), the AR-15 apparently never existed in the first place, which raises the question of how it was inserted into the story.