I think it's basically a distinction without a difference. A semi-automatic gun fires a bullet as fast as the trigger can be pulled and released. That's a couple of rounds per second. I've fired an M-16 on full auto, and as I recall it fires about 5 or 6 rounds per second. And I can't see a substantial difference in terms of reasonable use. So I can't see any reason whatever that any citizen would need an automatic weapon as opposed to a semi-automatic weapon. And I have not heard anyone offer any reason except for some paranoid fantasies about armed revolutions and armageddon (again, the lone warrior protecting his own, and dealing out his own brand of justice, his own way; the classic American male mythological hero). So I think we all agree that these weapons should be illegal, and are except for the gaping loopholes and the total lack of enforcement.
So next comes the semi-automatic assault rifles. Does anyone actually need one of those for home protection? Probably not. But I think this aspect of the debate misses the real goal. The goal is not to deny responsible citizens the right to protect themselves with firearms. The goal is to keep firearms away from irresponsible citizens. So the real questions we should be asking, is how best do we do that?
And the answers begin with way we enforce social responsibility with the use of other potentially deadly machinery. We teach, we test, investigate, and we license. And then we punish those who don't comply. You want to own a semi-automatic assault weapon, you can. But first you have to get a license. And then you have to prove that you know when, and how to use it, and when and how not to. Then you must accept, and pass, scrutiny, and agree to follow these rules, thereafter.