Pistol caliber carbines fill a niche between handgun and rifle, serving as personal defense weapons in the hands of professionals who don’t need to carry a rifle and as utility guns and home defense guns in the hands of private citizens. The new Model 9 from STAG Arms marries the ubiquitous 9mm to the proven and reliable STAG platform.

The mag well has been modified to utilize colt-style magazines, the ejection port has been shortened, and an oversized brass deflector has...