Why would they need to be illegal? with a mag change taking less than .5 second to swap even GI 30 round mags, is a 100 round mag really going to make much difference? We are not talking about a full-auto weapon in this case, so rate of fire is not going to change by much. Capacity is really pretty meaningless in the grand scheme of things. Really more of a distraction tactic used to scare folk with the mental picture of some nutjob running around blasting away at full autoburn.
The weapon in question most are concerned with is not a military weapon, it is a military style weapon and there is a difference. I firmly believe the 2ND amendment protects the citizens right to be at least as well armed as the military. It helps to make sure those asking these questions understand that NO LEGALLY BOUGHT full-auto weapon has ever been used in a commission of a crime in this country since the passing of the law controlling them. It also helps if you can gently explain to them that owning a machine gun is absolutely legal, assuming you purchase the Tax Stamp as required by law and follow the procedure for purchasing.
Have found it helps if you gently remind them that much like a woman will do with shoes or handbags etc. many collect firearms in the same way. Asking a woman why she needs 100 pairs of shoes that all look the same to many people will get you not much more than a funny look and a "because I like them" etc. I personally have 400 or so 1/32 slot cars and a 100 foot track in my basement, and when asked why I reply pretty much the same, I like them and they are one of my hobbies. A gun enthusiast is no different, so why the need to explain why one may have guns they have never fired, or a whole rack of historic military or western or whatever guns? I am sure theres a guy out there that has a wild hair to own at least one of every polymer pistol produced and has a collection that would make no sense to many, and you know what? THATS HIS EFFIN RIGHT. And you know what else? Until he decides to pick one of those pistols up and commit a crime the law supports his right to do so and as an AMERICAN citizen it is the duty of every single American to support and protect that right, with just as much or more effort as protecting free speech or religious freedom or any other right that might seem less dangerous or make more sense to you personally.

