To be fair there was talk that he actually had some training, I remember there being some talk yesterday he'd had some military experience. Which could explain why he was quite so ruthless with the firearms.... There was indeed, but I thought (that I read somewhere that) the norwegian authorities said that he had been exempted from national service. (though can't find any links due to internet being full of repeat reports)
But to go round shooting 80 odd takes a little bit of skill and a lot of mental preparation.