That was one thing I have been trying to find out as well, whether or not I could use my leftover 30 round mags AS LONG AS I alter the internals to make them hold only 10 rounds.
And yes it is OPTIONAL to register firearms in NJ. I am not sure if it has/is/or will be changed to be mandatory or not, as I have heard that was the case.
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00 T/A NBM
04 Silverado SS - Sold 
00 Trans Am WS6 NBM - Sold
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