AR15 would be the most common simply because it has near perfect ergonomics, inherent accuracy, modularity and mountains of aftermarket parts.
As far as the this gun is more dangerous than that gun debate - it's a load of shit. To limit the amount of damage done by someone going postal you would have to restrict us all to muzzle loaders. That's about as much as I'm saying on that matter, before this thread gets that "aw jeez not this shit again!" vibe.
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