When people talk about regular citizens not needing 30 round magazines and AR15's, we need to ask them why the police need them. The response from anti's will be something along the lines of "they need them because they have to face heavily armed criminals".
At that point, we say, "why? Are heavily armed criminals seeking out the police to shoot at them?" When the anti replies, we must lead him/her to say that the police are after the criminals because they did something to law abiding citizens. We can then say, "why can't citizens have the same rifles and magazines to use against the criminals that the police do? The citizens are the ones facing the heavily armed criminals first."