I must say that there's a fair bit of input here from people who (or so it seems to me) have absolutely no experience with firearms.
Pou's statement above is pretty fair, however. The AR-15 is a small calibre rifle (I have a .22, but I've also got a .270 which puts that to shame) and it's perfectly legal for me to own a sidearm (pistol) that does significantly more damage.
Theoretically (because I would never, ever do this), with what I can legally own here in Australia, I could take two handguns and my .270 and clean up any densely populated area (school, Westfields etc.).
There are a whole bunch of problems in the US outside of the actual firearms. How many of the shooters are part of the pharmaceutical culture of the US, and how is this affecting them? You need to go to a deep, dark place for a long period of time before you carry out something like a school shooting, so why isn't mental health the primary concern?
America has had bucketloads of guns since forever, so why have the mass shootings only happened over the last 30 years, since the pharma industry took off? Sure, the firearms available now are more developed than in the past, but like I said above you can achieve a pretty sick result with what we can legally own here in Australia.
Having said all of that, I can't argue against Australia's system. I need a police check, and the police can come into my house basically whenever they want. Each firearm needs to be registered and stored correctly at all times. I treat this system with respect and it works for me.
On the flip side, my country wasn't founded on an armed uprising against the government, so I guess I don't have that in my blood.