If your in Jersey the mags have to be welded to 15 or fewer rounds to be legal. The catcher though is that unconverted magazines aren't allowed into the state, so buying hi-caps at a gun show then bringing them home with the intention of converting them is a big no-no.
The other side of this issue is if you buy one of those AR-15 or AK mags that looks like a 30 rounder, but only holds 10 or 15, buy a couple true 10 rounders for the range. Why you may ask? Because you'll spend 75% of your range time explaining to every Billy, Bob, Range Officer Joe, lil Bubba, and maybe even that nervous police officer over there that they only hold the legal limit and they are legal. It's a pain in the butt that isn't worth it.