Most people who aren't particularly knowledgable about firearms think that when the politicians are talking about a gun or magazine ban, they are talking about confiscating those already in existence, as well as banning new ones. They think in terms of "all we have to do is wave a magic wand and ban high capacity magazines, and they vanish from existence". So, first off, straighten your friend out on that account. Confiscation is not a feasible solution--there is no way to know who owns a high capacity magazine, and only (some) law abiding citizens will voluntarily hand over their magazines. Criminals will definitely keep theirs. The fed wont pass a high-profile law they know they can't possibly enforce.
Then I'd note that most gun murders don't happen in mass shootings.
Most gun murders also involve the illegal drug trade in one way or another, not school children. The Newtown massacre actually shows how ineffective gun control is in stopping gun violence. Connecticut is ranked by the Brady Campaign as having the 5th strictest gun control in the country-- including an assault weapons ban and waiting period. All of this did nothing to prevent the shootings. Ask your friend what new gun restriction would do more to protect the people slain than allowing a few teachers who had concealed carry permits to be allowed to carry on school property that day?
As for his "muskets, not machine guns" argument, it would help to put things in context. American citizens were not prohibited from owning any arms of any kind, up to and including those that were equal to or even superior to what the British used during the Revolutionary War. Private citizens owned warships, cannons, as well as muskets. Which would be the equivalent of you owning a fighter jet and Patriot missles today.