The actual NJ law provides that you must be in possession of an FPID whenever you are in possession of a firearm, unless you fall within one of the enumerated exceptions.
The enumerated exceptions practically swallow the rule that you are required to possess an FPID when possessing a firearm such as: (going to and from hunting--no FPID required, going to and from a licensed gun range-no FPID required, going to or from gun smith, no FPID required, possessing in or about your residence or place of business-no FPID required.)
The problem you will face is that many people (as exemplified by the responses above) DO NOT KNOW THE LAW. I have been stopped hunting multiple time by PD's and they demand my FPID along with my license-it's not required, but I would rather show it to them instead of going to jail becaus ethey didn't know the law.
You run into fuzzy areas with the whole thing when discussing activity that is asside from the enumerated exceptions to the possession law. If you are going hunting and stop for breakfast at the dinner-do you need an FPID? the law says no need for the FPID "to and from hunting"-but is stopping at the Dinner "to and from hunting" or somthing else? Likewise, you purchased your new SKS and want to show it off to your best buddy-that's not in the exceptions at all-you best have your FPID with you.
Finally, as to the original poster-sounds like shooting at your friends property is not to or from an official shooting range-I would carry my FPID in your case.