The region in England he lived (very rural and with a large and famous national park) suggests that he might well have been a shooting club member, but the weapons don't sound legal. A doctor who inspected the first two people to die said that he hadn't seen shotgun wounds like that before, so it may have been modified.
Apparently he did have the license for them guns according to some reports but yes they were some crazy guns. Locallers claimed he was a nice and quiet guy, lived alone (not suprising) and he knew some of the ones he shot, maybe theres a motive behind it?
Agree with JBF and Fred, had this guy been a muslim I would have liked to have known how this one turned out in the media and investigation.