I don't really like it when people get all technical about the purpose of guns, or how technically a gun's purpose is to propel a piece of lead at high velocity, etc, etc, smart aleck remarks ad nauseam.
The most important purpose of a gun is to take life, human or otherwise. Depending on the situation, but mostly today they are important for taking human life because we have the taking animal life thing down to an assembly line procedure. The other stuff, the games and fun stuff, are still based on hitting a target, and the skill that is developed is useful for taking life. If the purpose of a gun were truly divorced from the skill of taking life, if it were just fun to propel lead downrange, we wouldn't even use targets would we?
We shouldn't argue that guns aren't for killing, because they are. We should just explain that people still kill and get killed without guns; guns give the elderly, the weak, females, and others a better chance of survival. Also, guns do not MAKE people kill other people (despite the fact that killing has been their primary historical purpose).
Learn to argue for self-defense rights and this whole "guns are for killing" debate becomes a moot point. If you argue it on the terms of the opponent, in this case, you lose, because guns are for killing. Do not reduce the debate to a semantic heap of nonsense, simply argue that killing can be justified and that, for the foreseeable future, the gun is the best option for the honest person to use for self-defense.