On a semi auto shotgun the max magazine capacity is 6 (plus the one in the chamber). You cannot have a pistol grip on a semi auto shotty at all, no matter the capacity.
There are no restrictions on pump action. Pistol grip - OK. Enormous magazine - OK too.
I have a Benelli SuperNova with a 24" barrel and 9+1 capacity. The magazine is about an inch shorter than the barrel.
So of I had a choice 18 20 22 24.
20 to 22 may be the sweet spot? I'm looking at the benelli m2 for this.
Craziest ,thing thing today I was talking shotguns with my local ffl today and he mentioned 10 rounds was ok in NJ. I said I dont know about that I think its 6. He insisted 10 as long as it had. I pistol grip etc. 10 seems way out of whack to me. I didnt even know what to say especially in another man's store. So I left it alone. In belleville there was another ffl that rhymes with mullet hole that told me I can but 17 round mags with my glock 17 and he would of given them to me as I'm retired leo. Even I know not to go near that. Alot of misinformation out there .