I'm not a lawyer, so please don't use what I say as an authoritative interpretation of NJ law, but...
NJ law is grey. There's no black and white, and this gives prosecutors leeway. Under NJ law, "large capacity magazines" are illegal and defined as anything that feeds a semi or full auto firearm continuously and holds greater than 15 rounds.
The NJ administrative code goes further to elaborate on ways in which hi cap mags might be considered non-large capacity magazines. This includes permanently altering the mag to never again hold greater than 15 rounds. It doesn't say how to do that, but it does say that "pins" and "blocks" that *could* be reversed do NOT render an otherwise hi cap mag legal. What does that mean? I don't really know. It means whatever you do to make a 16 rounder into a 15 rounder can not possibly be undone without destroying or seriously altering the mag, I assume. It also means you have to be really, really good at welding. Mags are aluminum right? I've heard that's extra tricky. And if the LEO who is picky about mag capacity can figure out a way to make it hold 16, you're in trouble. It also means you need to do these alterations out of state, since having them in your possession is a crime.
I almost had a smith alter the standard 16 rounders for me, but decided to just get the 10s. I hate the little plastic nubs but use them for the range anyway. Then I bought some 15 rounders for $25 on sale from CZUSA (check it out if you haven't). So much easier. And if the LEO asks about capacity I have a receipt that says "15 round magazine."
