Necessity, they say, is the mother of invention. And the machinations of politicians make invention necessary more often than gun owners would like. But, fortunately for the firearms industry, sometimes the new inventions are simple and inexpensive like the AR-15 Receiver Spur from S&B Products.

Passed on the heels of the Sandy Hook Elementary school shooting in Newtown, Conn., the New York Secure Ammunition and Firearms Enforcement Act of 2013 included many provisions that...