That's because they don't know anything about them. I've got five words for you: "shoulder thing that goes up."
Oddly, I've had some pretty decent luck (at least seemingly) with some liberal fence-sitters who were leaning anti-gun simply through education. Society has changed, and the average citizen doesn't seem to really know anything about guns anymore. I've run into more than a couple people who supported an "assault weapon ban" (and thus would be more likely to support a politician who voted for one) because they thought it was banning fully automatic (or burst) rifles. They see a picture of an AR-15 and don't realize that the model commonly sold to civilians is only semi-auto.
Then some will suggest that semi-automatic fire isn't needed, and you can talk about the many purposes of semi-automatic fire aside from military or criminal use. Same for magazine capacity (though this is arguably the hardest sell).
The key is being able to do so without coming across as a gigantic...well, word I can't say here (rhymes loosely with vassal), or a raving loon. But it can be done. At least some of the time.
May not keep them from voting for anti-gun politicians, mind you. For somebody who doesn't even know what an AWB is all about it's not likely to be a make-or-break issue for them...other things will come first. But hey, at least they won't blindly buy the anti-gun propaganda, and if someday they have the option to vote for a pro-gun Democrat or a generally liberal Republican, the Brady Campaign might be less likely to scare them off.