^^^Unless it's an assault weapon (defined by NJ law), you shouldn't have any worries about the legality of the gun itself. The FFL you ship it to would have to confiscate any magazines over 15 rounds. Or, you could keep them yourself and send two magazines with lower capacity (which is likely what the FFL would do anyway, then resell them to someone who can legally have them).
If it's a handgun, there aren't any NJ laws that I can find that prohibit handguns based on action type or anything else, unlike certain states that ban Glock-style pistols.