Wrong. Several styles available,
Hexmag Series 2 AR-15 .223 / 5.56 15/30-Round Polymer Magazine
A favorite among new and experienced tactical and sport shooters across North America, the Hexmag® Series 2 AR-15 .223/5.56 15/30-Round Polymer Magazine is a durable, yet lightweight aftermarket centerfire rifle magazine, designed to deliver superigunmagwarehouse.com
ASC AR-15 .223 / 5.56mm 15/30-Round Stainless Steel Magazine
Improving on the classic USGI AR-15 magazine, the ASC® AR-15 .223/5.56mm 15/30-Round Stainless Steel Magazine offers enhanced durability and longevity that’s hard to beat. Featuring stainless steel construction with a corrosion-resistant finigunmagwarehouse.com
Popular in NJ .
Yep.
For most anything it's probably trivial to block a mag down to lower capacity.
ARs are stupid popular with anything under the sun available more or less. What I'd be worried about though are proprietary guns, then how exactly the rule is worded - like if you change capacity on a Sig 226 mag via hardware store components to get under a limit does that count as a legal magazine? This kinda stuff comes into play in places with no grandfathering.
It's all total bullshit, just saying practically if you live there, there are concerns with how to deal with shit to avoid becoming a felon etc etc