K.I. S. S. in a nice way.
Machine guns are covered by other laws, dating back to the 1930s. The AWB does not legalize machinguns. Eliminate potential confusion right away.
It had little to no effect on criminals, especially those who are barred from purchasing or posessing any firearm. Criminals don't follow the law, check for a nod of comprehension.
Legislating away the Amendments is BAD, mmmmkay
Supporters deliberatly use misinformation and hyperbole to advance their agenda.
Guns, particularily scary black rifles, are inanimate objects. They are not designed for killing, anymore than automobiles and hammers are. Use the Ted Kennedy line if appropriate.
Educate them if possible, pointing out how many full-cap mags and pre-bans you own might not be the best starting point. Pointing out that the DC snipers could have used deer rifle might not help either. The question about children and guns in the home shouldn't be refuted with a snide remark about bad parenting.
I especially like the comment about politicians making a fuss to say they did something.
Then after all the calm and logical stuff, tell them the sunset is just in time for pumpkin season