The slippery slope toward confiscation.
Remind him that gun owners despise violence, especially gun violence, for all the right reasons. The terrible loss of life, the anguish of the parents, the aching hole that all violence leaves in the victims. As gun owners, as human beings, as parents, brothers and sisters, we all deplore violence, but that murder and mayhem are already against the law.
Then ask your friend if he's okay with the government confiscating private property that until yesterday was legal to own? Ask your friend how well the other government confiscation (illegal drugs) is working? Ask your friend if he's okay with giving the government permission to take private property from law-abiding citizens whose only problem is that they own something that until yesterday was completely legal?
Remind your friend that a magazine is simply a metal box with a spring inside. That it's easily manufactured in an industrial society, and that there are tens of millions of them in circulation today. Remind him, also, that after WWII, the Japanese government collected hundreds of thousands of swords and destroyed them, yet today it's easy to buy a correct antique samurai sword on the market. (Those swords with artistic merit were spared) Obviously such a ban didn't work in Japan, and it won't work here. The analogy doesn't work perfectly, but it is close. Bans simply don't work, except against the law-abiding.
Remind your friend also about the latest David Gregory incident, where he held up a magazine on national TV, in a city where those magazines are banned. Remind your friend that the law should apply to everyone, regardless of social standing and if he believes that Gregory shouldn't be prosecuted for simply possessing the magazine in a city where that magazine is banned, then he should re-examine his perceptions about the rule of law.
Gun control isn't about guns, it is about control. I don't want the government to have that kind of control over me.