My usuall line is that it didn't effectively do anything to stop criminals use of these weapons, and the statistics from Brady Campaign and Handgun Control Inc are not reliable. I then talk about how you are going to listen to a POLITICIAN about an issue, and believe everything they say. Most people will concede the politicians can be some of the biggest liars, and this helps destroy someones credibility.
Also I go onto that more 'assault weapons' have been produced and sold in the last 10 years the ban has been in effect. Despite that violent crimes used with guns as a whole have gone down, and crimes involving assault weapons hasn't moved from its '3%' statistic, that is basically too small to accurately measure. Somewhere in here I also point out that NOONE has ever been tried for breaking the ban.
If they ask about why police chiefs all seem to be rallying around in support of the ban, I inform them that they are appointed officials and as such have their own agendas.
For good measure I usually throw in the regular old lines about how we already have laws against murder, and that doesn't stop it from happening and my favorite:
"Does a piece of paper between a criminal and an assault rifle TRULEY make you feel more secure?"