I don't know of any gun shop in NJ that will sell you a non-compliant weapon or magazine, whether they are selling you their product or acting as FFL for an out-of-state purchase.
Since you have the FPID and Handgun Purchase Permit, you can buy as many compliant rifles and shotguns as you like, but you can only buy one handgun with the permit (there are special circumstances for a matched pistol set). And even if you have multiple permits, you can only make one purchase every 30 days.
Pretty much everything from a slingshot, bow, crossbow, to an AR or AK is considered a firearm. Pistol provisions are pretty simple. There's a max length, no secondary grips or shoulder stocks, and no mags that hold more than 10 rounds.
Rifles have a min barrel and overall length, cannot have a threaded barrel or flash arrestor. A welded on muzzle break is OK. No collapsible stocks Same mag limit, but while hunting 5 rounds is the max.
"The upper class: keeps all of the money, pays none of the taxes. The middle class: pays all of the taxes, does all of the work. The poor are there...just to scare the shit out of the middle class."--George Carlin