I think the poster before who suggested avoiding the argument of needs said it best. I think the best way to accentuate that is to go down the car, or food route.
Meaning....why do you need all this food?
To feed my family.
Do you need this much?
Because we would lose weight.
Could you live at a lower weight?
So you need less food. Let me have this food. (Pick out steaks and beer)
No, I bought it...I need it.
No, you need some food.
Maybe you could give your entire food budget to the government and they give you back something with nutritional value in the amount they determine you need.
How is this related to 30 round mags?
Guns, yes the entire gun, including mags, ammo, sling, etc are all protected rights by the second amendment. Food, especially the good food is an unprotected product that you have NO fundamental right to. If we accept removal of rights, how do we protect ourselves from a government who might try to manipulate us with food.
Point out a dictatorship who's dictator is full after each of his 3 squares, but many of his people starve to death. Really, you can point to most countries on the globe. You could point to the USA. Diverting corn to fuel makes people not afford food in the USA everyday.
Most likely you will be done.
If not, point out how the New York law prevents police from having more than 7rds. The police are going nuts and the governor is making laws with his mouth to make 30 rd mags ok for police....hmm wish he support his constituents like that. Wonder why the police and his security team need 30 rnd mags?