My own tack is to reference things like the Bath, MI school bombing, Beslan, Oklahoma City, 9/11, and other such events to show that when dealing with mass murder, firearms are for amateurs. As long as nutjobs can mix up some incendiaries and/or explosives from common household chemicals, they will find ways to commit mass murder. An AWB doesn't help one whit.
A second tack is to reference the drug war. After all, cocaine and heroin are illegal, but anyone can get some fairly easily, albeit expensively. And then I go into the Poles mass-producing STENs under German occupation in WWII, the Australian crime syndicate that was discovered running a factory producing integrally-suppressed Owen SMGs, Pakistan's home-built gun trade, and the IRA's crude SMG factories during the Troubles. Anyone with a machine shop can make a functional, albeit crude, SMG. With advances in 3D printing coming in the next decade, look for it to become even easier to produce a functional SMG with minimal talent and just a Google search.
The point I try to make is that, as far as guns go, the genie is out of the bottle. You can't ever close Pandora's box. Even if you could, there are far more devastating methods and tools out there, and if we could magically remove every 'assault weapon' from the US, it would just lead to higher body counts when the crazies start tossing Molotovs and pipe bombs or placing IEDs under school buses. At least you have a chance to tackle or shoot a gunman.