Was watching a video review. He said "you can't put any kind of grip up here otherwise it becomes an SBR." (He lives in PA by the way)

 

A handgrip configuration determines SBR loopholes? What did he mean by that?

 

Also, if Rifle evil features were neutered on this gun would it be NJ legal or do we have other restraints too.?

Did he say grip? The issue with these is when you take the Sig arm brace and instead if attaching it to your forearm, you shoulder it and use the brace as a stock. This is where the ATF has come out and said you are illegally in possession of an SBR if used in this manner.

He probably meant to say AOW (any other weapon), instead of SBR.  A pistol with more than one vertical grip is classified as an AOW by the ATF.  Same process to register with the ATF, but a $5 tax stamp vs $200 tax stamp.

He probably meant to say AOW (any other weapon),

 

THIS! 

I bought an MPX-P on Friday.  I have a can I'm waiting on a stamp for.  I'll get another stamp and SBR it with a collapsible stock that SIG makes for it. 

He was referring to a forearm grip. He said. "You can only have a finger stop up here "

 

So what's the deal with these? Rifle or handgun?

Civilians in free states can purchase these as pistols. If they want to SBR it they can file a form 1 for a tax stamp and make it a rifle.

He was referring to a forearm grip. He said. "You can only have a finger stop up here "

 

So what's the deal with these? Rifle or handgun?

They are considered pistols/handguns, since there is no stock present. If you SBR it, different story.

The MPX-P is a pistol.