The bottom line here is that any regulations of the types of firearms , or accessories, is still infringement of the right to bear arms.
The debate of how morality cannot be regulated can go 'round and 'round.
Do I, personally, think that 100 round magazines are necessary? No. I actually think that most hi capacity magazines are junk, jam and waste more ammo than they fire in a lot of cases.
My every day carry is a 1930's Smith & Wesson .32 caliber revolver (used to be standard NYPD issue back in the day) It is not a "man stopper" as a 9mm or .45 would be, but I keep it loaded with hollow point rounds. It is one of the most accurate revolvers that I have ever had the pleasure of firing. For personal or family self-defense, I think it would suffice just fine and I hope to never have to use it. If I couldn't hit my target with 5 of the 6 rounds, I would use the 6th round on myself for lack of target discipline. I doubt that I would ever need to have more than those 6 rounds in most any situation where I would have to defend myself barring a zombie apocalypse. It is ancient, accurate and reliable.
However, the 2nd amendment is not really geared towards self-defense from criminal intent but rather to defend one's self in the event of a tyrannical government. In that case, my 6 rounds wouldn't amount to much. A lot of folks feel the same way and prefer higher capacity magazines. I spent many years behind an M-16 using 30 round magazines. It does not take much time to change them and they are fairly easy to carry. It is what I would prefer.
Just because I prefer them or see no particular advantage to high capacity magazines, it does not mean that I am against others owning them.
If someone has intent to kill another human being or a group of human beings, they will find a way whether it be through the use of high capacity magazines, rocks, flame throwers, grenades, poison or the simple and intentional spread of disease. Limiting one route will just make them resort to another. Humans are a very ingenious bunch.