The issue is one of efficacy: the capacity to produce an effect. If two possible drugs were on the market to treat a disease and one has a 25% success rate and the other a 90% success rate, would it make sense to prefer one over the other? Yes, of course, one is clearly more effective (higher efficacy).
Guns also have a high efficacy. That's why people feel so strongly about having them as a tool for self-defense--they work really well for the purpose. But, it works the same in reverse. They are also very effective at killing. Look at mortality rates for different methods of suicides for instance and guns are up there at the top. Treating guns the same as all other tools and weapons is quite simply stupid. It'd be no different then a doctor saying both of those drugs, previously mentioned, are exactly the same.
It makes sense to treat weapons/devices/tools which are particularly dangerous differently and we do it all the time. It takes much more training and licensing to drive a semi truck versus a car, for instance. And then again it takes even more if you transport flammable/hazardous materials. That makes sense doesn't it?
That is the theory behind bans on assault weapons and large magazines. Both of those items have a higher efficacy when it comes to slaughtering fleshy creatures.