You are correct about the problem with entering the "need" discussion. The problem with not entering that discussion is that it leaves on answered the challenge of the assertion "no one needs a high-capacity magazine, or more than 10 rounds for legitimate self-defense".
The "need" discussion can be an eye opener.
Start by talking about illegal deaths involving guns being balanced by the positive effects of firearms. Briefly mention self-defense, national security (remember Yamamoto's quote from WWII about a rifle behind every blade of grass and the Swiss national defense paradigm), note that they provide some level of defense against tyranny, note the deterrent value of an armed society in discouraging certain types of crimes (e.g. burglary of predominantly occupied homes in "gun-free" societies vs. burglary of mostly unoccupied homes in "gun-rich" areas).
You may want to touch on the idea that the "equalizing" factor provided by firearms has a civilizing effect on society by elevating it from the state where the biggest, strongest person rules by virtue of nothing other than his physical prowess to a level where democracy is possible and decisions must be discussed because even the physically weak and unimposing can't be ignored.
Then you note that it is hard to quantify those benefits and state that the easiest way to justify firearms is via their recreational value--and pause to give them a little rope.
When they jump on that one, interrupt and ask them about the need for drinking alcohol, owning jetskis, or trampolines. We, as a society, tolerate the decidedly negative aspects that these things (and other similar and common things) often bring in terms of death and injury in spite of the fact that the only real point in their favor is that they provide recreation--people like them and want them.
If society can turn a blind eye to DWI deaths in the name of recreation, why is anyone even daring to talk about restricting firearms when everyone knows that they provide important and quantifiable benefits beyond recreation.