Is it legal in NJ to own a 10+1 mag fed pump action such as mossberg 590?

Yes - 100%.  No restrictions on pump action shotguns except for the federal ones (minimum barrel length/oal/etc)

Yes, Mossberg 590M is legal. Since the magazines are unique do to the fact that they do not fit in any semi auto gun, you can get the 15 round and 20 round magazines also. 

Why stop at 10?  Do I hear 20?  YES, sold to that Eagle Scout right there!  

22 minutes ago, Smokin .50 said:

Why stop at 10?  Do I hear 20?  YES, sold to that Eagle Scout right there!  

https://adaptivetactical.com/products/venom-10-rnd-box-kit-shotgun-magazine-conversion-kit-with-10-rnd-box-mag-and-wraptor-forend

What if I told you I converted a Black Aces Pro Series M mag fed Semi-Auto shotgun to take Mossberg 590 mags? 

15 minutes ago, PK90 said:

What if I told you I converted a Black Aces Pro Series M mag fed Semi-Auto shotgun to take Mossberg 590 mags? 

 

 

1 hour ago, PK90 said:

What if I told you I converted a Black Aces Pro Series M mag fed Semi-Auto shotgun to take Mossberg 590 mags?