Kel-tec SU16 fills the same niche as the original Colt AR15: a lightweight but capable rifle that can go anywhere and not unduly burden the person carrying it. With 18.5″ barrel, it weighs less than an M1 carbine or even the 14.5″-barreled M4 carbine. It has a greater sight radius than AR15 of the same length, and the forend unfolds into a bipod. It’s not intended as a military weapon but can be pressed into defensive use and perform competently. The variant shown is the A model updated with metal front sight base.
My own go-to .223 rifle is an SU16E picked for reliability and lack of blowback with a sound suppressor. The barrel is lighter than that of most AR15s, but civilian firefights involving more than a hundred rounds are pretty rare.