I always use a pretty well-rehearsed point when somebody says "I don't think people need assault rifles".
I start out with something like... "Well, lets just say we're not talking about guns now. Let's say the gov't tried to pass a law saying that you couldn't buy a diesel pickup truck unless you had something that weighed 10k lbs to tow with it... or that you couldn't buy a house with more than 1 bedroom unless you had kids... or that people who live in apartments couldn't own dogs above a certain weight... or pick something they own that you can use as an example.
A normal person would think it's pretty silly or perhaps even downright dumb to buy a gigantic truck just to drive around town. A person would never say though, "I wish they'd pass a law so that everybody has to drive Honda Civics if they work in an office building because "nobody needs" a big truck unless they work in construction".
When you talk about "what somebody needs" outside of the context of guns, you can usually see the light come on in their heads about how dangerous it can be if the gov't starts to dictate what "somebody needs", because their thoughts will drift to all the absurd **** they own that they don't need. It becomes personal to them instead of "somebody elses problem". You can almost always get somebody to admit that "I don't want the gov't telling me what I can and can't own, but I still think nobody needs an assault rifle." At that point, you've at least got them thinking about it, and can progress into a talk about just how fantastic a self-defense weapon an AR is, or talk about the other hunting and sporting applications... they'll listen to you now that their mind isn't completely stuck on "I don't like them".