20” HBAR 1x9 twist barrel attached to aircraft quality aluminum
upper with A-2 Length rifle hand guards and comes with your choice of an
A-2 or flat-top upper. Comes with everything to turn your stripped lower
receiver into a standard A-2 rifle.

This AR-15 Rifle kit is headspaced, and ready to mount to your lower receiver.  This kit includes all parts except the Lower receiver.  Once you have assembled YOUR lower with the parts provided in the kit, and mount the upper,  this is an operational firearm 5.56mm.

5.56mm NATO chamber information here.  All barrels made for Mil-Spec 5.56mm NATO