From Wikipedia:
http://en.wikipedia.org/wiki/2011_Norway_attacksHe hollowed out the rear seats of his Hyundai Atos in order to have enough space for the firearms he hoped to buy. After two days, he bought professional materials about mining, which were supposed to give him an alibi in case someone suspected him of preparing a terrorist attack.<79> He wanted to buy an AK-47 assault rifle (this firearm is however not common in the country, unlike Vz. 58), a Glock pistol, hand-grenades and a Rocket-propelled grenade, stating that getting the latter two would be a "bonus". Contrary to his expectations, he was completely unable to get any firearms in the Czech Republic, and noted that it would likely be easier to buy illegal weapons in Western European cities. Following this failure, he decided to obtain a semi-automatic rifle and a Glock pistol legally in Norway, noting that he had a "clean criminal record, hunting license, and a pump action shotgun Benelli Nova already for seven years", and thus obtaining the guns legally should not be a problem.<77>
Back in Norway, Breivik obtained a legal permit for Ruger Mini semi-automatic carbine, ostensibly for the purpose of hunting deer. He bought it in autumn 2010 for € 1,400. Getting a permit for the pistol proved more difficult, as he had to demonstrate a regular attendance at a sport shooting club.<79> He was able to acquire a suppressor for the Ruger, and bought 30 magazines from a US supplier. In November, December and January he went through 15 training sessions at the Oslo Pistol Club, and by mid-January his application to purchase a Glock 16 pistol was approved.<80>So, it seems now that the shooter in the Norway attacks purchased his firearms legally, using a standard semi-automatic hunting rifle and a standard semi-automatic pistol. He bought his weapons one to seven years prior to the attack, had permits for them, and belonged to a sport shooting club to satisfy the requirements of the law.
What can we infer from this?
Waiting periods, mandatory registration and/or permits, and sporting club memberships won't stop anyone who is committed to a course of action.