For handguns and rifles, 15 rounds is your limit. Permanently modified magazines are okay, but in many cases you can just find 15 round magazines, so you should go with that when possible to avoid any issues. [The gun shop near me sells the S&W M&P 15-22, and it comes from the factory with a 25rd mag that is permanently modified to take only 10 rounds (it's designed for multiple states' compliance, hence the 10rd mark).]
For a semi-auto shotgun, the round count is much lower. You can have only up to 6 rounds in a semi-auto shotgun (frankly, you'd be hard pressed to find a semi-auto shotgun that would take more than that without a tube extension anyway). That 6 round limit does not apply to pump-action shotguns. There are OTHER rules that also apply to shotguns; you can check with your local dealer as they know what is legal and what isn't.
For semi-auto rifles, like an AR-15, there are a whole host of things you can't have in NJ that you can have elsewhere. Hence, you'll find that manufacturer's often make what they call a "compliant" version of their AR's. This is a version that usually has the following details: FIXED in place stock (many times this means a "pinned in place" collapsible stock (so it's no longer collapsible); 10rd magazine (again, to be in compliance with a bunch of states, you can buy 15rd mags later); no bayonet lugs; no flash hiders -- most 'compliant' rifles will have a very plain ending barrel, with no threads (you can have compensators, but they must be permanently attached).
The best advice I can offer is do NOT buy a rifle or shotgun from a person or shop that originates out of state unless you are absolutely certain it is legal in NJ. You should play it safe and buy rifles/shotguns directly from a NJ FFL, and even then check to see that the actual model they are selling you is the properly 'compliant' one. While you can hop over to Cabela's in Pennsylvania to buy a long-gun (with your NJ FPID), I would bet that they would not even know if the gun they are selling you will be legal in NJ or not. It's quite likely that in many cases (especially AR-15s, since most come with adjustable stocks!) the model they have is a standard model and not compliant in NJ.
Last edited by jg0001; May 22, 2010 at 10:37 PM.