I find this very interesting.
Last year when I was looking for 20rnd straight mags for service rifle competition, I found out that the magazine capacity is 15 rounds max in NJ. So I had to buy straight, 10 rnd mags from Bushmaster. I was OK with that. At the time, I thought I read that the mag cap for handguns was 12 rounds.
Now, looking at NJ Statutes 2C:39-1, Section Y, any firearm (... any handgun, rifle, shotgun, machine gun, automatic or semi-automatic rifle...) cannot have a magazine capacity of more than 15 rounds.
I thank you all for pointing this out. For some reason I had the number 12 stuck in my head. I guess the resellers I contacted don't know this either so they didn't point that out.