I would offer the following argument to gun banners, which - as JohnKSA noted - they would find unpursuasive if their minds are already decided.
If they are thoughtful, however, they may consider the matter more deeply:
The best answer to the question: "Why does any citizen need an AR-type rifle with 30 round magazines?" was actually provided by Abraham Lincoln who said: "This country, with its institutions, belongs to the people who inhabit it. Whenever they shall grow weary of the existing government, they can exercise their constitutional right of amending it or their revolutionary right to dismember or overthrow it" (Abraham Lincoln, April 4, 1861).
100 years later Lincoln's statement was echoed by Democratic Senator Hubert Humphrey, who clarified: "...the right of the citizens to bear arms is just one guarantee against arbitrary government and one more safeguard against a tyranny which now appears remote in America, but which historically has proved to be always possible" (Humphrey, 1960).
The Second Amendment exists because an armed citizenry is our last, best hope of ensuring, again in Lincoln's words: "...that government of the people, by the people, for the people shall not perish from the earth".
That is why responsible citizens should own AR-type rifles and 30 round magazines. No one needs more than 10 rounds to hunt deer. But one needs more than ten rounds to overthrow a government.
This is the 800 lb gorilla sitting in the living room being marginalized by many on both sides of the issue.
Treat everyone you meet with dignity and respect....but have a plan to kill them just in case.