You presume that mass killings often include an assault weapon, rifle. This is sometimes the case but never a requirement.
To make an accurate and complete characterization of the decision leading to choose an an assault weapon, rifle, you need to consider all of the possible reasons for that choice. Let me highlight the aspects of the decision of which I'm alluding. You see someone who plans a heinously evil act, choosing such a gun and you infer (based on that choice) that attributes of the weapon have determined his choice. The "assault weapon" line of thought, the BS ban ideas relating to their use in crime and especially mass shootings and singling semi-auto ARs and AKs, in particular, is wholly invalid, materially distracting and essentially works to undermine the idea of gun laws influencing crime at all.
It is entirely possible that some mass shooters (criminal, evil, and/or crazy) choose a rifle like an AR-15 for same reason lots of non-criminal, non-evil and non-crazy (regular) folks do. Rather than try to explain why regular choose the AR, I'll refer you to the thread that covers that: http://www.democraticunderground.com/1172196149
I mentioned that you asked a two part question and the beginning of my reply addressed the implied portion. The explicit question, (Why do we see so few mass killings by fully automatic machine guns if legality doesn't stop criminal mass killers?) has a simple answer. I have been told that generally there are two very useful functions of full-auto that can't be addressed as efficiently with a semi-auto. The first is suppressive fire. This is entirely useless to a mass shooter since just about all of them choose venues where folks are unarmed. If you choose a venue where no one is armed, there is no hostile fire needing suppression. Generally suppressive fire is employed by group. These gutless mall-ninjas tend to work alone. The second scenario where full-auto shines is in 'clear the room' tasks. This is where the task at hand is to neutralize a room full of hostiles who will return fire. As I already said, these cowards generally choose disarmed victims. A room full of disarmed victims means one can take his time shooting and killing them all.
Eventually these useless turds are confronted by armed resistance (usually police) at which time they are shot, shoot themselves or are captured. They mostly lack any exit strategy other than suicide (sometimes suicide by cop) or the hope fame and interviews by the BAU and the media.
Somewhere you mentioned that fertilizer access was now more restricted than it used to be. That's true but ammonium nitrate/fuel type explosives are only one of numerous combinations of readily available substances which could be used as explosive components or provide the raw materials for explosive components.
Guns were never as common in the UK as they were here but they have become even more scarce and some mass killers have moved on to arson as their means.
It is not the law or law enforcement that will by force mold society into a less violent image of itself. Education, alternatives, support and empowerment are the means to reduce violence.
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