I just quickly searched through the "New Jersey Administrative Code Title 13, Chapter 54. Firearms and Weapons (N.J.A.C. 13:54-1 et seq.)" document and only saw references to 15 rounds, not 10, so maybe I'm thinking of something else.
I think that limitation was in the Federal law that has expired. More info regarding the laws can be found on the NJ State Police web site under 'Firearms'. The 15 round magazine limit also applies to handguns. For shotguns, it is 6 rounds.