The firearm and munitions kit is a cartridge and a barrel insert for a firearm adapted to fire munitions comprising a 223 round. The cartridge has an axis, a neck, a shoulder, a body, an extraction groove, and a slight frustoconical shape extending axially from the widest body diameter to the beginning of the shoulder. The barrel insert is appropriately chambered and provided for operability with the cartridge and round. A chamber of the barrel insert has an axis, a neck bore, a shoulder bore, and a body bore. The shoulder and the shoulder bore are formed at an angle of approximately 29.degree., .+-.0.5.degree. with respect to the axis of the chamber. Upon firing a firearm equipped with the barrel insert and munitions of the present invention, the round is capable of reaching a minimum velocity of greater than 2,000 fps.