Linus and his Merry Men (aka H4)
Argument from authority. And anyway, these were people living in the early post-revolutionary USA, who had, a bit more than a decade before, fought a war against one of the world's biggest superpowers.
People here in this thread are not going into some very important issues about guns. Reading this, it's like if guns were the product of some domestically grown plant or whatever. The truth is, guns are built by big companies for a profit. For a profit means they are willing to sell to almost anyone. For example, to third world dictatorial goverments that use them to arm death squads against peasants. In fact, the US government has been this whole century heavily involved in that (think El Salvador, Guatemala, Indonesia and others, armed by the US).
I think most USians have never seen armed official repression first hand. I for one, have luckily only had a really slight taste of it, which involved no shooting, just sharpshooters aiming rifles from helicopters at unarmed students for no good reason; just to make a show of force, film the whole thing, edit it, and make some propaganda about "students attacking our democratic ideals". Political persecution, that's it. (The pro-statehood party in Puerto Rico has a long history of political persecution against independentists. Kidnapping, murder, infilitration, and public massacres in my university included).
The point being, it's easy to defend the right of USians (and conveniently ignoring the rest of the world) to bear arms (and therefore defending the arms industry) while sitting comfortable in front of a computer, reading /., getting flown around first-class to give conferences, and, in general, close to the top of the food chain, claiming that you need to defend yourself from those in the bottom of said chain.
Just thinking about the immediate issue of 'self-defense' is failing to grasp the big picture, where a few privileged few who enjoy huge power, which the people did not appoint to, benefit from all this misery. Think Indonesia and Guatemala again: governmental death squads, armed by the US, massacre people to defend the interests of huge corporations.
I figure I'm just ranting by now. So I'll come to a close. The best reasons for opposing arms are, IMHO, antimilitarism and pacifism in general. This does not mean opposing the right to defend oneself against criminal aggression in the USA; it means opposing the purported right of a select, non-elected few to profit from widespread misery, all over the world.